What should I do if my compressed images still fail Lighthouse scores?

If your images are already compressed but Lighthouse still scores them poorly, the issue is usually not just file size. Lighthouse looks at a few image-related factors, so here’s what to check next:

1) Serve the right image dimensions

A common problem is sending images that are much larger than the displayed size.

• If an image displays at 400x300, don’t serve a 2000x1500 file.
• Use responsive images (srcset, sizes) so browsers can pick the best size.

2) Use modern formats

JPEG/PNG can still be heavier than needed.

• Prefer WebP or AVIF when possible.
• Keep JPEG only when needed for compatibility or certain photo workflows.

3) Lazy-load below-the-fold images

If images aren’t visible right away, they can still hurt Lighthouse if they load too early.

• Add loading="lazy" to non-critical images.
• Don’t lazy-load the main hero/LCP image.

4) Optimize the LCP image specifically

Lighthouse often penalizes the largest contentful paint image if it loads slowly.

• Make sure the hero image is:
  • properly sized,
  • preloaded if necessary,
  • not lazy-loaded,
  • served from a fast CDN.

5) Check image delivery performance

Even a small image can score poorly if delivery is slow.

• Use a CDN
• Enable caching
• Compress over the wire with Brotli/Gzip where applicable
• Make sure response times are low

6) Avoid unnecessary image processing overhead

Sometimes Lighthouse flags things because images are decoded or rendered inefficiently.

• Avoid huge CSS backgrounds if a regular <img> works better
• Minimize layout shifts caused by missing width/height
• Set width and height attributes to reserve space

7) Inspect Lighthouse’s exact warning

Lighthouse usually tells you why.

Look for messages like:

• “Properly size images”
• “Serve images in next-gen formats”
• “Defer offscreen images”
• “Largest Contentful Paint image was lazily loaded”

That will point to the actual fix.

8) Re-check after fixing

After changes:

• Clear cache
• Run Lighthouse again in incognito
• Test on mobile throttling, since that’s often the stricter result

If you want, I can help you interpret a specific Lighthouse warning or audit screenshot and tell you exactly what to change.