Which format should you choose?
JPEG is the most compatible format for photos: every device, program and website reads it, but it does not support transparency. PNG is lossless: it keeps every pixel and the transparency, which suits screenshots, diagrams and logos, but it is heavy for a photo. WebP, designed for the web, usually gives lighter files than JPEG at similar quality and keeps transparency; all recent browsers read it, some older programs do not.
What the conversion changes in the image
The image keeps its dimensions and its orientation: a photo taken in portrait comes out the right way up. Its metadata (date, camera, GPS location) is removed, and the tool says so. The color profile of a JPEG, PNG or WebP is kept; an image read by the browser (HEIC, AVIF, GIF, BMP, TIFF) is brought into sRGB, the standard color space of the web. When converting to JPEG, transparent areas turn white. An animated image becomes a still image: only the first frame is kept.
A conversion that stays on your device
Images are converted by your browser, on your device, with the libraries of Google's Squoosh app: MozJPEG for JPEG, libwebp for WebP, oxipng for PNG. They are never uploaded to a server and nothing is kept once the page is closed. The tool is therefore also suited to personal photos and confidential documents.
HEIC, AVIF and other formats
JPEG, PNG and WebP are read by the tool's own libraries, in every browser. GIF, BMP, AVIF, HEIC and TIFF are read by the browser itself: recent browsers read GIF, BMP and AVIF, but only Safari reads iPhone HEIC photos and TIFF. Elsewhere, the tool tells you it cannot read the file. On an iPhone, the camera's "Most Compatible" setting saves photos directly as JPEG.