What we check
We run Google Lighthouse on a simulated phone and collect its image audits, each with the specific files involved and the estimated bytes you would save.
- Modern formats: images that would be much smaller as WebP or AVIF.
- Responsive sizing: images downloaded at a larger size than they are displayed.
- Compression: JPEGs that could be re-encoded at a lower quality setting without a visible difference.
- Offscreen images that load immediately instead of when the visitor scrolls to them.
- Images without explicit width and height, which cause layout shift.
- A lazy-loaded hero image, which delays Largest Contentful Paint.
Why images are the quickest win
Firm websites often use large, high-resolution photos: office interiors, city skylines, team portraits uploaded straight from a camera at several megabytes each. The browser has to download the full file even when it displays it in a 400-pixel card. Fixing this rarely changes how the site looks, but it can cut page weight dramatically and improve loading time on phones.
WebP is supported by all current major browsers, and AVIF by almost all of them. Both usually produce noticeably smaller files than JPEG at comparable quality, which is why Lighthouse flags old formats.
How to fix it
Most CMSs can do the work automatically. WordPress generates several sizes for each upload and outputs srcset so the browser picks the right one; image optimisation plugins or your CDN can convert to WebP or AVIF on the fly. On custom sites, export images at twice their display size at most, compress them, and use the picture element to offer modern formats with a JPEG fallback.
Add loading="lazy" to images below the first screen, but never to the hero image — give that one fetchpriority="high" instead. Always set width and height attributes so the browser can reserve space. Finally, give every meaningful image a short alt text describing it; decorative images get an empty alt.
Workflow for the future
Fixing today's images is half the job; preventing new heavy ones is the other half. Agree on a simple rule for whoever uploads content: resize photos to at most about 2000 pixels wide before upload and let the CMS or CDN handle formats and sizes. Check new pages with this audit before announcing them.