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Before You Fix It: What This Check Means
Image format and sizing choices often dominate transfer weight on content-heavy pages. In plain terms, this tells you whether users are likely to feel this page as fast, stable, and usable. Scavo inspects image URLs from `<img>` and `<source>` elements, plus `srcset`, then classifies formats.
Why this matters in practice: degraded speed here often compounds into lower conversion and weaker user trust.
How to use this result: treat this as directional evidence, not final truth. Synthetic sampling may not fully match real-user device/network variability. First, confirm the issue in live output: verify directly in live production output with browser/network tools Then ship one controlled change: Use an image build pipeline/CDN that auto-generates AVIF/WebP derivatives. Finally, re-scan the same URL to confirm the result improves.
Background sources
TL;DR: Images are in unoptimised formats or not properly compressed, adding unnecessary weight to every page load.
Images comprise 21% of total webpage weight on average, with pages loading 4.4MB of image data (HTTP Archive, 2024). Only 30% of images are served in WebP format despite 97% browser support (W3Techs, 2026) — meaning most sites are still serving larger JPEG/PNG files when a smaller, identical-quality alternative exists. Switching to WebP typically cuts image bytes by 30-50%.
What Scavo checks (plain English)
Scavo inspects image URLs from <img> and <source> elements, plus srcset, then classifies formats.
What it treats as modern:
.avif.webp.jxl<source type="image/avif|image/webp|image/jxl">hints
How results are assigned:
Info: no measurable image references foundPass: no legacy formats detected, or modern formats are at least ~60% of detected referencesWarning: legacy formats dominate
Important detail: this check is about format signals, not exact image byte size (payload size is measured by image_payload_budget).
How Scavo scores this check
Scavo assigns one result state for this check on the tested page:
- Pass: baseline signals for this check were found.
- Warning: partial coverage or risk signals were found and should be reviewed.
- Fail: required signals were missing or risky behavior was confirmed.
- Info: Scavo could not gather enough reliable evidence on this run to score pass/fail confidently.
In your scan report, this appears under What failed / What needs attention / What is working for image_optimization, followed by Recommended next steps and Technical evidence (for developers) when needed.
- Scan key:
image_optimization - Category:
PERFORMANCE
Why fixing this matters
Modern formats can deliver meaningful byte savings at similar visual quality. That generally improves LCP behavior, mobile experience, and bandwidth efficiency.
In growth terms: lighter visual payloads reduce first-impression friction on landing pages where conversion intent is fragile.
If you are not technical
- Ask for a list of your top 10 image-heavy templates (home, pricing, blog, docs).
- Ask whether your image pipeline can output AVIF/WebP automatically.
- Prioritize the largest above-the-fold images first.
- Re-run Scavo and confirm modern-format percentage improves.
Technical handoff message
Copy and share this with your developer.
Scavo flagged Image optimization (image_optimization). Please migrate dominant legacy JPG/PNG/GIF delivery to modern formats (AVIF/WebP where supported), keep fallbacks, and share before/after examples on key templates.If you are technical
- Use an image build pipeline/CDN that auto-generates AVIF/WebP derivatives.
- Serve modern formats with
<picture>and safe fallbacks where needed. - Keep source assets high quality, but ship responsive variants by viewport/device density.
- Avoid accidental format regressions from CMS uploads or editor bypass paths.
- Track adoption by template class, not just globally.
How to verify
- Inspect rendered HTML for modern
src/srcset/<source type=...>usage. - Confirm legacy-heavy templates now serve modern variants.
- Spot-check in Chrome/Safari/Firefox for fallback compatibility.
- Re-run Scavo and compare
modern_format_percentandlegacy_examples.
What this scan cannot confirm
- It does not score perceptual image quality.
- It does not directly measure bytes transferred (use
image_payload_budgetfor that). - It cannot fully validate browser-specific codec support behavior in every environment.
Owner checklist
- [ ] Assign one owner for image delivery standards.
- [ ] Enforce modern format generation in CI/CD or CDN transforms.
- [ ] Add regression checks for new CMS-uploaded assets.
- [ ] Re-test top conversion templates after image pipeline updates.
FAQ
Should we convert everything to AVIF only?
Usually no. Use AVIF/WebP with graceful fallbacks to maximize compatibility.
If we use WebP already, is that enough?
Often a strong baseline. AVIF may still reduce bytes further for many assets.
Can this pass while a few legacy images remain?
Yes. The check passes when modern coverage is strong enough, but large legacy hero assets can still hurt real performance.
Where should we start?
Start with above-the-fold images on high-traffic pages.
Sources
- web.dev: Choose the right image format
- web.dev: Learn images
- MDN:
<picture>element - MDN: Responsive images guide
Need a format migration plan by template priority? Send support your top landing and product URLs.
