What image format should I use for SEO?

WebP is the best format for most images. It is 25-35% smaller than JPEG at equivalent quality and 25-35% smaller than PNG for images with transparency. All modern browsers support WebP. Use JPEG for photographs where file size is critical. Use PNG only when you need lossless quality or complex transparency. Use SVG for logos, icons and simple graphics.

Does alt text help with SEO?

Yes, in two ways. First, Google uses alt text to understand and index images — pages with properly described images appear in Google Images searches, driving additional traffic. Second, alt text is a weak relevance signal for the page's topic. Pages with keyword-rich alt text that accurately describes the images have a marginal ranking advantage over pages with missing or empty alt text.

What is lazy loading and should I use it?

Lazy loading delays loading images until they are about to enter the viewport as the user scrolls. This reduces initial page weight and speeds up LCP for above-the-fold content. Add loading='lazy' to all img tags except the first image visible on the page — that image should load immediately with loading='eager' and fetchpriority='high' to optimise LCP.

Image SEO: Formats, Alt Text, Compression and Loading

Images are the most common cause of slow pages and one of the most overlooked SEO opportunities. Properly optimised images improve your Core Web Vitals, rank in Google Images, and make your pages load faster for every user.

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How images affect your SEO

Images affect SEO in three ways: page speed, Google Images rankings, and content relevance signals. Large unoptimised images are the single most common cause of poor LCP scores. Missing alt text means your images are invisible to Google Images. Keyword-relevant alt text reinforces your page's topical focus.

Issue	SEO impact	Fix
Large file size	Slow LCP, poor Core Web Vitals	Convert to WebP, compress
Missing alt text	Not indexed in Google Images	Add descriptive alt attribute
No width/height	Layout shift increases CLS	Add explicit dimensions
No lazy loading	Unnecessary bandwidth on load	Add loading="lazy"
Wrong format (BMP, TIFF)	Unnecessarily large files	Convert to WebP or JPEG

How to optimise images step by step

Run the Image Optimisation Auditor on your key pages
Fix missing alt text — add descriptive alt to every image that conveys information
Add width and height attributes to every img tag to prevent CLS
Convert JPEG and PNG files to WebP using Squoosh.app or your build pipeline
Add loading="lazy" to all images below the fold
Add fetchpriority="high" to your main hero image to prioritise LCP
Re-run the Page Speed Analyzer to confirm LCP improvement

Biggest impact: Converting your hero image to WebP and adding the correct dimensions typically improves LCP by 30-50% on image-heavy pages. Do this first before any other optimisation.

🖼 Audit Images Now

Run the Image Optimisation Auditor and get actionable results in minutes. Pay as you go — no subscription needed.

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Image SEO: Formats, Alt Text, Compression and Loading

How images affect your SEO

How to optimise images step by step

🖼 Audit Images Now

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