How to Optimise Images for Search Engines?

Last updated: September 2026

How to optimise images for search engines: file formats, compression, alt text, filenames, responsive sizing and the checks that protect your rankings.

Optimising images for search engines means reducing file size, choosing the right format, writing descriptive filenames and alt text, and serving the correct dimensions for each screen. Image optimisation matters because images are the heaviest part of a typical web page and usually the element that decides how fast the page feels.

Key takeaways

• Images are the single largest contributor to page weight. On the median mobile home page they account for more bytes than JavaScript, fonts, CSS and HTML put together, according to HTTP Archive’s 2025 Web Almanac.
• Your biggest image is very often the element Google measures for Largest Contentful Paint, which means one oversized hero photo can fail a Core Web Vital on its own.
• Format choice does most of the work. At the 90th percentile, a JPG on a desktop home page weighs 278 KB where an AVIF weighs 37 KB.
• Alt text is the most important piece of metadata you control on an image, and in Australia it carries an accessibility obligation as well as an SEO benefit.
• Renaming, compressing and sizing an image takes under a minute, and almost nobody does it.

What does image optimisation for SEO actually mean?

Image optimisation is the work of making every image on a page as light as it can be while still looking right, and making sure search engines can tell what the image contains.

Those are two separate jobs that get confused constantly. Compression, format choice and responsive sizing are performance work, and they affect how fast the page loads. Filenames, alt text, captions and structured data are meaning work, and they affect whether the image can rank in Google Images and whether the page reads as relevant.

Doing only one half is the most common mistake. A beautifully described image that weighs 3 MB still slows the page down. A tiny, fast image called DSC_0481.jpg still tells a search engine nothing. Both jobs take seconds each and both need doing on every image you upload.

There is a third job now too. AI search tools reading your page use alt text to understand what your images show, because they cannot see the picture either. Written descriptions are doing more work in 2026 than they were doing in 2022.

Why do images matter for search rankings?

Images matter because they dominate page weight, and page weight drives the performance signals Google measures.

HTTP Archive’s 2025 Web Almanac, published in January 2026, found that the median mobile home page ships 911 KB of images, compared with 632 KB of JavaScript, 122 KB of fonts, 77 KB of CSS and 22 KB of HTML. Images outweigh everything else on the page combined. The same chapter found the median home page loads 19 separate images.

That weight shows up in a metric Google uses directly. In Google’s own documentation, a good Largest Contentful Paint is 2.5 seconds or less, anything over 4 seconds is classified as poor, the measurement is taken at the 75th percentile of page loads, and <img> elements are among the element types eligible to be the LCP element.

Read those two facts together and the practical picture appears. On most pages, the largest visible thing is a photograph. That photograph is what Google times. If your hero image is a 2 MB JPG straight out of a camera, you are not failing Core Web Vitals because of your theme or your hosting. You are failing because of one file.

There is a second, simpler reason images matter. Google Images is a search surface in its own right, and product, recipe, property and trades businesses pick up real traffic there. An image that has been named and described properly can rank. One that has not, cannot.

Which image format should you use?

Use AVIF or WebP for photographs, PNG for logos and graphics that need transparency or hard edges, and SVG for icons, charts and anything drawn rather than photographed.

The size difference between formats is not marginal. The 2025 Web Almanac measured image sizes on desktop home pages by format and found that at the 90th percentile, JPG files reached 278 KB and PNG files 278 KB, while WebP reached 97 KB and AVIF reached 37 KB. Same visual job, a fraction of the bytes.

Figures in the table are drawn from the 2025 Web Almanac’s measurement of desktop home page images by format, and describe what the heaviest tenth of real-world files weigh, not a target you should aim for.

One point about SVG worth pausing on. Text inside an SVG stays selectable and machine-readable, which means a chart drawn as SVG can be read by a search engine and by an AI assistant. The same chart exported as a PNG is a picture of numbers and nothing more. If you publish data, draw it.

How small should your images be?

Aim to keep individual content images under 150 KB and a hero image under 250 KB, and resize every file to the largest size it will actually be displayed at before you upload it.

That second half matters more than compression. A photo from a modern phone is often 4,000 pixels wide. If it sits in a content column that is 800 pixels wide, three quarters of the data you served is thrown away by the browser. WordPress will display it at the right size, which hides the problem completely, because the visitor still downloads the original.

The order of operations is: crop, resize, then compress, then upload. Doing it in that order means the compression works on an image that is already the right shape, and you are not compressing pixels you were about to discard.

Do not compress the same file twice. Keep the original somewhere and export a new version when you need a different size, rather than re-saving the web copy over and over.

How do you write alt text that works?

Write alt text that describes what the image shows, in a plain sentence, as though you were describing it to someone on the phone.

Google’s image documentation is direct about this. Google names alt text as the most important attribute for providing metadata about an image, says it should be useful and information-rich and use keywords in the context of the page content, and warns that filling alt attributes with keywords is keyword stuffing that may cause a site to be seen as spam.

In Australia there is a second reason to get this right. The Australian Human Rights Commission publishes Guidelines for Equal Access to Digital Goods and Services, issued under section 67(1)(k) of the Disability Discrimination Act 1992, which point Australian organisations toward the Web Content Accessibility Guidelines. The guidelines are not legally binding on their own, but they are the benchmark the Commission works from, and providing a text alternative for non-text content is the first requirement in WCAG.

A screen reader user hearing “image” or “DSC underscore zero four eight one dot jpg” has been given nothing. That is the actual test.

Keep alt text to roughly one sentence. If the image genuinely needs a paragraph to explain, that paragraph belongs on the page as visible text, where every visitor benefits from it.

How should you name image files?

Name the file for what is in it, in lowercase, with hyphens between words, before you upload it.

Google’s guidance is explicit here too: the filename gives Google clues about the subject matter of the image, short but descriptive filenames are preferred, and the documentation contrasts my-new-black-kitten.jpg favourably against IMG00023.JPG while advising against generic names like image1.jpg and pic.gif.

So hot-water-system-replacement-clayton.webp rather than IMG_4417.jpg. Hyphens rather than underscores, because hyphens are read as word separators. Lowercase throughout, because some servers treat capitals as a different file.

Rename before uploading, not after. Once WordPress has generated its thumbnail sizes and inserted the image into posts, renaming means broken images and redirects you did not need to create.

Do image captions help?

Captions help readers more than they help rankings, and reader attention is the part you should care about.

A caption sits under the image, is visible to everyone, and tends to get read even by people skimming. That makes it a good place for a genuinely useful sentence: what the reader is looking at, why it matters, what the number means.

The rule is the one Google applies everywhere. Write the caption if it adds something. Do not add a caption to every image just to create another place to put a keyword, because a page dotted with forced captions reads as a page written for a machine.

How do you serve the right image on every screen?

Serve responsive images, set explicit width and height attributes, and lazy load anything below the fold.

Three things to get right, and WordPress handles most of them if you let it.

Responsive sizing. The srcset attribute lets the browser pick the file that suits the screen it is on, so a phone does not download the desktop version. WordPress adds srcset automatically to images inserted through the editor. Images hardcoded into a theme or page builder often miss out, which is where most problems live.

Width and height attributes. Setting these on the <img> tag lets the browser reserve the right space before the image arrives, which stops the page jumping as it loads. That jump is measured as Cumulative Layout Shift, another Core Web Vital.

Lazy loading, but not everywhere. Images below the fold should load only when the visitor scrolls to them. Your hero image should not. Lazy loading the image at the top of the page delays the very element Google is timing for LCP, which makes the score worse rather than better. In WordPress this means excluding the featured image and the first in-content image from whatever lazy loading plugin you use.

Should images go in your sitemap and your schema?

Add images to your sitemap if your plugin does it for you, and add image structured data only where the page type genuinely calls for it.

On a WordPress site running Rank Math or Yoast, images are already included in the XML sitemap by default. There is rarely a reason to build a separate image sitemap by hand. Check that the setting is on, then move on to something that matters more.

Structured data is different. Product, recipe and article schema all include an image field, and filling it in properly is what lets your image appear alongside a rich result. Adding image licence metadata is worth it if you publish original photography and want it credited in Google Images. For a standard blog post on a small business site, setting the featured image so the article schema has an image value is the whole job.

What does an image workflow look like for a Melbourne business?

Picture a landscaping business in Bentleigh. The owner photographs every finished job on his phone, uploads eight or nine shots straight into a gallery on the project page, and the page now weighs 14 MB. On a phone on mobile data it takes so long to render that most people leave before seeing a single garden.

The photos are the best asset the business has. The way they are being used is costing it work.

Here is what DigiFix would do.

Week 1: audit which pages are worst, run them through PageSpeed Insights, and identify the LCP element on each.

Weeks 1 to 2: set up a repeatable process, meaning the owner crops and resizes on the phone before uploading, and the site converts to WebP automatically on upload.

Weeks 2 to 4: work back through the existing library, replacing the heaviest files first and renaming and describing as each one is replaced.

Ongoing: check Search Console for image impressions, since a landscaper’s before-and-after shots should be earning traffic from Google Images.

How do you check whether your images are hurting your site?

Run the page through PageSpeed Insights, find the LCP element, and start there.

PageSpeed Insights names the specific element it timed for Largest Contentful Paint, and lists oversized and uncompressed images as separate opportunities with the kilobytes each one is costing you. That gives you a ranked list rather than a vague instruction to compress things.

Google Search Console gives you the other half. Filter the performance report by Google Images to see whether your pictures are earning impressions at all. A trades or retail business with hundreds of original photos and no image impressions has a naming and describing problem, not a photography problem.

Fix in this order, because the return drops sharply as you go down the list.

Where the images themselves come from is a separate question with its own set of licensing traps, and DigiFix covers it in the guide to free stock photo sites for blogging. Getting the topic right before you worry about either is covered in the DigiFix guide to choosing a topic for your blog.

What do Australian businesses ask about image SEO?

Where should your business go next?

If you have never audited your images, start with your homepage and your two best-performing service pages, since that is where the traffic already is. If you publish regularly, fix the upload process first, because every week you do not, you add more files to clean up later.

The DigiFix blog covers the rest of the technical and content side. Finished work is in the DigiFix portfolio, and you can meet the people who would do the work on the About DigiFix page.