Image Format Optimization (WebP and AVIF)

April 3, 2026

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Why Image Format Matters

Images are typically the heaviest assets on a WordPress page. On an average WooCommerce product page, images account for 50–80% of the total page weight. The format those images are stored in directly determines how large the files are — and how long visitors wait for them to appear.

JPEG and PNG have been the default image formats on the web for decades. They work everywhere, but they are not efficient by modern standards. WebP and AVIF — newer formats designed specifically for the web — produce significantly smaller files at the same visual quality.

Mochyon Lightspeed detects this automatically. If your site serves images in older formats where modern alternatives would reduce file size, Mochyon Lightspeed flags them and estimates the potential savings.

The Formats Compared

JPEG is the most common format for photographs. It uses lossy compression — it discards visual information to reduce file size. JPEG has no transparency support. It has been the web standard since the 1990s.

PNG is used for graphics, logos, and images that need transparency. It uses lossless compression — no quality loss, but larger files. PNG photographs are dramatically larger than JPEG equivalents.

WebP was developed by Google and supports both lossy and lossless compression, plus transparency. A WebP image is typically 25–35% smaller than an equivalent JPEG and significantly smaller than PNG. WebP is supported by all modern browsers — Chrome, Firefox, Safari, and Edge.

AVIF is the newest format, based on the AV1 video codec. It achieves even better compression than WebP — typically 40–50% smaller than JPEG at equivalent quality. AVIF support is now available in Chrome, Firefox, and Safari. It is the most efficient option available, but encoding is slower and some older browsers do not support it yet.

How Image Format Affects Performance

Smaller image files download faster. On a mobile connection, the difference between a 400KB JPEG and a 250KB WebP version of the same image can mean 500ms or more of saved download time. When that image is the hero image that determines your LCP, the format choice directly affects your Core Web Vitals.

Across an entire page with multiple images — product galleries, category grids, blog post thumbnails — format savings compound. A page with 2MB of JPEG images might weigh only 1.2MB with WebP, reducing both total page weight and the bandwidth your visitors consume.

How WordPress Handles Image Formats

WordPress 5.8 and later generate WebP versions of uploaded images automatically when the server supports it (requires PHP’s GD library or Imagick with WebP support). However, this only applies to newly uploaded images — existing images in your media library remain in their original format.

For comprehensive format conversion, image optimization plugins handle both new and existing images. These plugins typically convert images to WebP on upload, generate WebP versions of existing images in bulk, and serve the appropriate format to each browser using <picture> elements or server-side content negotiation.

What to Look For

If your site’s images are served as JPEG or PNG and your visitors use modern browsers — which the vast majority do — you are delivering larger files than necessary. The savings from converting to WebP alone are substantial, and AVIF offers even more for browsers that support it.

The combination of modern image formats with proper compression settings and responsive image sizing can reduce image-related page weight by 60–70% compared to unoptimized JPEG/PNG originals.

Tools That Can Help

ShortPixel compresses and converts images to WebP and AVIF on upload, with bulk optimization for existing media libraries. It handles the format conversion and serves the right format to each browser automatically via its adaptive images feature.

Imagify (by the WP Rocket team) offers similar compression and WebP conversion with a straightforward interface. It integrates well with WP Rocket’s cache and CDN features for a coordinated approach to image delivery.

EWWW Image Optimizer can run entirely on your server without sending images to an external service — useful for sites with privacy requirements or very large media libraries.