Images are one of the most overlooked elements in SEO. Most optimisation guides focus on text content, backlinks, and technical structure — and images get a brief mention about alt text before moving on. But images affect your SEO in multiple ways simultaneously: they impact page speed, contribute to on-page relevance signals, and can drive significant traffic through Google Image Search when optimised correctly.

Given that images are often the single largest contributor to page weight and load time, image optimisation is also one of the fastest ways to improve your Core Web Vitals scores and by extension your rankings.

File Size and Format

Unoptimised images are the most common cause of poor page speed scores. A single high-resolution photograph saved directly from a camera or design tool can be several megabytes — and a page with five or six of these images can take ten or more seconds to load on mobile.

Use WebP format. WebP is a modern image format developed by Google that delivers significantly smaller file sizes than JPEG or PNG at equivalent visual quality. Converting your images to WebP typically reduces file size by 25-35% compared to JPEG. All modern browsers support WebP.

Compress before uploading. Use a tool like Squoosh (squoosh.app) or ShortPixel to compress images before they go onto your server. Target file sizes of under 100KB for most images and under 200KB for large hero images. Check how your images affect your overall load time using our page speed checker.

Match dimensions to display size. If an image displays at 800 pixels wide on your page, there is no benefit to uploading a 3000-pixel-wide version. The browser downloads the full file and then scales it down — wasteful bandwidth for both you and your visitor.

Alt Text

Alt text (alternative text) serves two purposes: it describes the image to visually impaired users using screen readers, and it tells search engines what the image depicts since they cannot see images directly.

Good alt text is descriptive and specific. Compare these:

  • Bad: alt="image" or alt="photo1"
  • Mediocre: alt="dog"
  • Good: alt="Golden retriever sitting on grass in a park"
  • SEO-optimised: alt="Broken link checker tool scanning website for 404 errors"

Include your target keyword naturally in alt text where it genuinely describes the image. Do not stuff keywords into alt text where they are irrelevant — this is a spam signal Google recognises.

Decorative images that add no informational value should have empty alt attributes: alt="". This tells screen readers to skip them, reducing noise for visually impaired users.

File Names

The filename of your image is another relevance signal Google uses. DSC_4471.jpg tells Google nothing. broken-link-checker-tool.jpg reinforces the topic of the page. Use descriptive, hyphenated filenames that reflect the image content and align with your page's target keyword.

Lazy Loading

Lazy loading delays the loading of images that are below the visible screen area until the user scrolls to them. This significantly improves initial page load time and LCP scores — as we covered in our guide to Core Web Vitals — because the browser only needs to load the images immediately visible when the page first opens.

In HTML, add loading="lazy" to any image that is not in the initial viewport. Do not use lazy loading on your hero image or any above-the-fold images — these should load immediately for the best LCP score.

Image Sitemaps

Google discovers images through crawling, but you can also submit an image sitemap to help Google find and index your images more efficiently. An image sitemap extension adds image-specific tags to your existing sitemap entries. This is particularly valuable for sites where images are a primary content type — photography portfolios, product catalogues, or news sites.

As discussed in our guide to building a sitemap, submitting your sitemap to Google Search Console accelerates indexing. Adding image data to that sitemap extends the same benefit to your visual content.

Structured Data for Images

Adding ImageObject schema markup to pages with important images helps Google understand the image context — what it depicts, who created it, and what page it belongs to. This can improve eligibility for rich results in Google Image Search, which drives additional click-through traffic.

Summary

Image optimisation delivers dual benefits: faster load times that improve Core Web Vitals and rankings, plus direct visibility in Google Image Search. Convert to WebP, compress before uploading, write descriptive alt text with natural keyword inclusion, use descriptive filenames, and implement lazy loading for below-the-fold images. These changes together can dramatically reduce your page weight and improve both rankings and user experience.

Missed the previous article? Read: How to Write SEO-Friendly URLs That Rank