---
title: "Image served in a legacy format (perf/image-legacy-format)"
description: "perf/image-legacy-format: AVIF and WebP are typically 25-50% smaller than JPEG/PNG at the same quality and are supported by every current browser."
canonical: https://void-design.vercel.app/rules/perf/image-legacy-format
lastModified: 2026-09-16
---

# Image served in a legacy format

`perf/image-legacy-format` · severity **warn** · category Performance · detected by `void perf` and `void audit`

## Why it matters

AVIF and WebP are typically 25-50% smaller than JPEG/PNG at the same quality and are supported by every current browser.

## How to fix it

Serve AVIF or WebP (content negotiation, <picture>, or next/image). Keep SVG for vector art.

## Example

```
<picture><source srcset="/hero.avif" type="image/avif"><img src="/hero.jpg" alt="…"></picture>
```

## More performance rules

`void perf` reports 25 rules in this category. Core Web Vitals and bytes: LCP, CLS, TBT, TTFB, JavaScript, CSS, fonts, images, caching and compression, measured as the median of several runs on a throttled mobile and a desktop profile.

- `lcp-image-lazy` LCP image is lazy-loaded — [perf/lcp-image-lazy](https://void-design.vercel.app/rules/perf/lcp-image-lazy)
- `lcp-image-no-priority` LCP image has no fetchpriority="high" — [perf/lcp-image-no-priority](https://void-design.vercel.app/rules/perf/lcp-image-no-priority)
- `lcp-background-image` LCP element is a CSS background image — [perf/lcp-background-image](https://void-design.vercel.app/rules/perf/lcp-background-image)
- `image-oversized` Image is much larger than its rendered size — [perf/image-oversized](https://void-design.vercel.app/rules/perf/image-oversized)
- `images-missing-dimensions` Images without width/height or aspect-ratio — [perf/images-missing-dimensions](https://void-design.vercel.app/rules/perf/images-missing-dimensions)
- `render-blocking` Render-blocking resources delay first paint — [perf/render-blocking](https://void-design.vercel.app/rules/perf/render-blocking)
- `no-text-compression` Text resources are served uncompressed — [perf/no-text-compression](https://void-design.vercel.app/rules/perf/no-text-compression)
- `cache-headers` Hashed static assets are not cached long-term — [perf/cache-headers](https://void-design.vercel.app/rules/perf/cache-headers)

Detected by `void perf` and `void audit`. Explain it in a terminal: `void rules perf/image-legacy-format`