Four formats cover essentially everything on the modern web, and each of them is the right answer to a different question. Converting between them is easy; knowing which direction is worth going, and which direction quietly costs you something, is the part that matters.
What each format is for
JPG is thirty years old and still the correct choice for photographs when compatibility matters more than bytes. It is lossy, it has no transparency, and every device made since the 1990s can open it. Nothing else is as universally safe.
PNG is lossless and supports transparency. That makes it right for logos, icons, screenshots, diagrams, and anything with hard edges or flat colour — content that JPEG handles badly. It is the wrong choice for photographs, where losslessness buys you a file several times larger for no visible gain.
WebP does both jobs. It has a lossy mode that beats JPEG at the same visual quality and a lossless mode with transparency that usually beats PNG. Every current browser supports it. Its weakness is outside the browser: some desktop applications and older devices still cannot open it, so it is a web format rather than an archival one.
AVIF is the newest and compresses hardest, particularly on clean graphic content and at lower quality settings. It supports transparency and wide colour. Browser support is now broad but application support lags further behind WebP's, and encoding it is slower. It is the right choice when file size is the priority and the destination is a web page.
The two conversions that cost you something
Converting a photograph to PNG feels safe because PNG is lossless, and it is a mistake. Lossless means the conversion does not introduce new loss — it does not recover the detail JPEG already discarded. You get a file that is often five to ten times larger, containing exactly the same visible image plus a faithful lossless record of the JPEG's compression artefacts. If the file is a photograph, it should stay in a lossy format.
Converting anything with transparency to JPG is the other one. JPG has no alpha channel, so the transparent regions have to become some colour, and that colour is almost always white. A logo designed to sit on a dark background comes back with a white box around it. The conversion is not wrong, but it is irreversible, and a tool should tell you it happened. Filum announces the flatten in the result rather than letting you discover it when you place the file.
There is also a non-conversion worth naming. Converting a JPG to a JPG is not a format change; it is a re-compression, and a second lossy pass over an already-lossy file. Filum declines same-format conversions and points you at Compress Image, which is the tool built for that job and which will tell you when there is nothing to gain.
How quality gets chosen
For lossy targets the question is what quality setting to encode at, and the honest answer is that it depends entirely on the image. A fixed number applied to every file overshoots on some and destroys others.
A better method is to encode at several settings, compare each result to the original with a perceptual similarity measure, and take the smallest file that still clears a quality floor. Filum works that way — you pick a named level, the encoder searches for the smallest file clearing that level's measured floor, and the actual measured score is shown with the result rather than implied. PNG output skips all of this, because it is lossless: every pixel identical to the decoded input.
One thing worth stating plainly, because the internet often says otherwise: newer formats are not uniformly smaller. Measured on a noisy photograph in July 2026, AVIF and a well-tuned JPEG encoder came out within a few percent of each other at matched quality — AVIF's advantage concentrates on clean and graphic content, not on grainy photographs. The per-file measured number is the claim worth trusting; a blanket ranking of formats is not.
Format detection, and where the conversion happens
A file's extension is a label, not a fact. Files get renamed, downloaded with the wrong suffix, or saved by software that guesses. A converter that trusts the extension will hand a PNG decoder a JPEG and fail with an error that blames your file. Filum reads the real format from the file's bytes and offers the other three formats as targets from there.
The conversion itself runs inside your browser, on your own device — the image is never transmitted to a server. That also means the format range is not limited by what a conversion service happens to sell: WebP and AVIF output are both available here, and when this was measured against the largest free image-tool suite in July 2026, neither WebP output nor AVIF in any direction was offered there at all.