Four formats cover almost every image on the web, and choosing well can cut a photo's size by more than half with no visible quality loss. Here's the short version.
Universally supported and fine for photographs, but it's old and inefficient by modern standards, and it can't store transparency. Use it when maximum compatibility matters more than file size.
Lossless, supports transparency, and stays crisp on logos, screenshots and line art. The downside is size: PNG photos are huge. Reach for PNG when you need a transparent background or pixel-perfect edges, not for photographs.
Supported by every current browser, WebP does both lossy and lossless, keeps transparency, and is typically 25–35% smaller than JPEG at the same quality. It's the easiest upgrade: swap JPEGs and PNGs for WebP and most pages get noticeably lighter with no visible difference.
AVIF is the newest and usually the smallest of all — often 50% smaller than JPEG — with excellent quality and transparency support. It's supported in all modern browsers now, but encoding is slower and a few very old clients can't open it. Use it when size is critical and you control where the image will be viewed.
pixelpix's image compressor and converter output all four formats with a live before/after slider, so you can watch the size and quality change and pick the winner — all in your browser, nothing uploaded. iPhone photos in HEIC convert too.