If you manage a website, you know the anxiety of a red score in Google PageSpeed Insights. The most common warning affecting your LCP (Largest Contentful Paint) score is: "Serve images in next-gen formats." It appears when your page contains JPEG or PNG images that could be dramatically smaller in WebP or AVIF format.
This guide explains exactly what the warning means, how WebP and AVIF compare, and — most importantly — the specific fix for WordPress, Shopify, and custom HTML sites, because the solution is different for each platform.
Quick Fix Summary — Serve Images in Next-Gen Formats
- ►What it means: JPEG/PNG images could be 25–90% smaller in WebP or AVIF, improving your LCP Core Web Vitals score
- ►WordPress: Upload WebP directly (5.8+ native), or use Imagify/ShortPixel for existing media library images
- ►Shopify: Nothing needed — CDN auto-converts to WebP for Chrome, Firefox, Edge, and modern Safari
- ►Custom HTML: Use <picture> with AVIF → WebP → JPEG fallback chain (full code in Section 3)
- ►WebP support: 97–98% globally in 2026 — safe as your primary format
- ►AVIF vs WebP: AVIF is 40–50% smaller than JPEG but ~93% browser support — best for large hero images only
- ►SEO impact: Fixing this improves LCP by 0.3–0.8s, moving from "Needs Improvement" to "Good" on Core Web Vitals
Convert free: PixelBatch WebP Converter — bulk convert offline, no uploads, no subscription.
1. What Does This Lighthouse Warning Actually Mean?
Google Lighthouse runs a performance audit of your page and identifies images that could reduce in file size by switching to a next-gen format. The warning triggers when Lighthouse estimates that converting your JPEG or PNG images to WebP or AVIF would save more than a few KB of total page weight.
This matters because large images are the most common cause of slow Largest Contentful Paint (LCP) — the Core Web Vitals metric that measures how long it takes for your page's main content to become visible. Google uses LCP as a direct search ranking signal.
What Counts as "Next-Gen" in 2026
Google Lighthouse considers WebP and AVIF to be next-gen formats. For 2026, the practical choice is between WebP (97-98% browser support) and AVIF (93% support).
2. WebP vs AVIF vs JPEG: The Data
| Format | Typical Size | Reduction vs JPEG | Browser Support |
|---|---|---|---|
| Original JPEG | 1,500 KB | Baseline | 100% |
| WebP (80% quality) | ~200-400 KB | 65-87% smaller | 97-98% |
| AVIF (80% quality) | ~150-300 KB | 73-90% smaller | ~93% |
| PNG | 2,000-5,000 KB | 33-233% larger | 100% |
Use our free WebP Converter with the visual comparison slider to verify quality before deploying to production.
3. How to Fix It — By Platform
WordPress
Option A: Plugin-Based (Existing Images)
Plugins like Imagify, ShortPixel, or WebP Express batch-convert and auto-serve WebP while keeping JPEG as fallback.
Note: Most have free tiers with monthly conversion limits.
Option B: Pre-Convert Before Upload (Cleaner)
Convert images to WebP using PixelBatch before uploading. WordPress 5.8+ accepts WebP natively. No plugin, no server-side processing.
Shopify
You don't need to do anything. Shopify's CDN automatically converts and serves WebP to supported browsers. If you still see the warning on a Shopify store, the issue is likely images in your theme's assets folder — convert those to WebP using PixelBatch and re-upload.
Custom HTML Sites
Use the HTML picture element with WebP source and JPEG fallback:
<picture>
<!-- AVIF: best compression for modern browsers -->
<source srcset="image.avif" type="image/avif">
<!-- WebP: fallback for browsers without AVIF -->
<source srcset="image.webp" type="image/webp">
<!-- JPEG: universal fallback -->
<img src="image.jpg" alt="Description" loading="lazy">
</picture>4. The Plugin Trap
Plugin Downsides
- Monthly subscription fees ($5–$20/mo)
- Upload limits on free tiers
- Server-side processing overhead
- Plugin abandonment risk
Pre-Convert Workflow
- Convert before uploading — free, no limits
- No server-side processing
- No dependency risk
- Works for any CMS
5. The SEO Impact
A 1.5MB JPEG hero image on mobile 4G takes approximately 1.2 seconds to download. The same image at 300KB WebP takes 0.24 seconds — nearly a full second faster. That single change can move an LCP score from "Needs Improvement" to "Good," directly improving your Core Web Vitals rating and search ranking.
Use our free WebP and AVIF Converter to bulk convert offline. Combine with our 100KB compressor to hit both the format and size targets.
Batch Convert Your Images to WebP Now
Drag and drop your entire folder of JPEGs. Convert to WebP or AVIF instantly — 100% offline, no uploads, no plugins, free forever.
Fix My PageSpeed ScoreFrequently Asked Questions
What does "Serve images in next-gen formats" mean?▼
This Lighthouse warning appears when your JPEG or PNG images could be significantly smaller in WebP or AVIF format. Large images cause slow LCP scores, which Google uses as a search ranking signal.
Does WebP work on Safari and iOS in 2026?▼
Yes. WebP has been fully supported by Safari since iOS 14 and macOS Big Sur. As of 2026, WebP has approximately 97-98% global browser support. Safe to use as your primary format.
Should I use AVIF or WebP?▼
WebP for most images — 97-98% browser support and 25-35% smaller than JPEG. AVIF for large hero images — up to 50% smaller but only 93% browser support. Use the picture element to serve both with a JPEG fallback.
How do I fix this warning in WordPress?▼
WordPress 5.8+ accepts WebP natively. For new images: convert to WebP before uploading using PixelBatch. For existing media library images: use Imagify or ShortPixel to batch-convert.
Does Shopify automatically serve WebP?▼
Yes. Shopify CDN automatically converts and serves WebP to Chrome, Firefox, Edge, and modern Safari. If you still see the warning, check images in your theme assets folder — these bypass Shopify's CDN conversion.
Does converting to WebP affect SEO?▼
Positively. Smaller WebP files improve LCP — a Core Web Vitals metric Google uses as a search ranking signal. Fixing this warning typically improves LCP by 0.3-0.8 seconds.