Choosing the right image format is one of the easiest ways to make files smaller and pages faster. But the choice isn't arbitrary — each format is good at different things. Here's a clear, practical comparison.
JPEG — the photographic workhorse
JPEG has been the standard for photographs for decades, and for good reason: it compresses continuous-tone images (photos, gradients) very efficiently. It's lossy, meaning it discards some detail to save space, and it does not support transparency. Use JPEG for photographs when maximum compatibility matters — every device and app on earth can open one. A modern JPEG encoder like MozJPEG can squeeze another ~10% out of a file at the same visual quality compared to older encoders.
PNG — sharp graphics and transparency
PNG is lossless and supports transparency, which makes it ideal for logos, icons, screenshots, diagrams, and anything with sharp edges or flat color. The trade-off: PNG is a poor choice for photographs — a photo saved as PNG can be several times larger than the same photo as a JPEG, with no visible benefit. Rule of thumb: graphics and transparency → PNG; photos → not PNG.
WebP — the modern all-rounder
WebP, developed by Google, supports both lossy and lossless modes and transparency. In practice it produces files roughly 25–35% smaller than JPEG at equivalent quality, and smaller than PNG for graphics. It's supported by every current browser. For most websites, converting images to WebP is a quick, safe win.
AVIF — the smallest, with a catch
AVIF is the newest of the four, based on the AV1 video codec. It offers the best compression by a wide margin — often 50% smaller than JPEG at the same quality, and it handles both photos and graphics with transparency. The catch is encoding speed (AVIF takes longer to create) and slightly less universal support than WebP, though all modern browsers now handle it. If page weight is your priority and your audience uses current browsers, AVIF is hard to beat.
Side-by-side summary
- Photograph, maximum compatibility: JPEG
- Logo, icon, screenshot, transparency: PNG (or lossless WebP)
- Best balance of size and support: WebP
- Smallest possible file, modern audience: AVIF
How much will you actually save?
On real photographic images, converting a large JPEG to WebP commonly saves 25–35%, and to AVIF often 50% or more — with no visible quality difference at sensible settings. For a website with dozens of images, that can mean the difference between a sluggish page and an instant one.
Converting between formats
slimdocs can both compress an image in place and convert it to another format in one step. Drop an image into the compressor, and under "Convert images to" choose WebP or AVIF to get the biggest savings. Everything happens in memory — your images are never stored on a server. You can also combine several images into a single PDF with the Images → PDF tool.