Image converter

Which of the three output formats you actually want

Only three formats can be written here, and the choice between them is usually decided by what is in the picture rather than by preference. PNG is lossless and stores flat colour and hard edges efficiently, which makes it right for screenshots, diagrams, logos, charts and anything containing text — and it is the only one of the three that keeps transparency unconditionally. JPG assumes smooth tonal gradients, so it is right for photographs, where it commonly reaches a tenth of the PNG size with no visible difference. Used the wrong way round, each is poor: a photograph as PNG is needlessly enormous, and a screenshot as JPG grows a grey halo along every letter.

WebP sits between them and is a delivery format rather than a working one. It keeps transparency like PNG while compressing photographs more tightly than JPG, which is why sites serve it — but far less software will open it, so it is the wrong choice for a file you intend to send to somebody. Convert to WebP for a web page you control, and keep the original.

Why AVIF, GIF and BMP can be read here but never written

The list above is deliberately lopsided: gif-to-png exists, png-to-gif does not. Everything on this site runs in the browser, and a browser canvas can decode a wide range of formats while being able to encode only PNG, JPEG and WebP. There is no AVIF, GIF or BMP encoder to call. Adding one would mean shipping a WebAssembly codec and uploading nothing still, but at a cost of megabytes per visit — which is a poor trade for formats a person rarely needs to produce.

In practice the asymmetry matches what people want anyway. AVIF and WebP arrive from websites and need to become something ordinary software will open; BMP arrives from old Windows tools and scanners and needs to become something smaller; GIF arrives as a still frame someone wants as a picture. The traffic runs one way.