Choose JPG, PNG or WebP for the job
JPG suits many opaque photographs and widely accepted upload forms. PNG is useful for transparency and sharp graphics. WebP is worth comparing for supported web publishing. Conversion alone does not crop the picture, reduce its width or restore missing detail, so choose a separate resize or crop step when needed.
A practical comparison before publishing
Try two outputs from the same original and compare them at the size people will actually see. Look at file size, fine lettering, skin tones and edges. A compact photo is useful only if its appearance remains acceptable. Keep your original so another format can be created without re-encoding an already converted copy.
If a file will not open
Supported inputs include JPG, PNG, WebP, GIF, BMP and AVIF where your browser can decode them. A recognised extension is not enough if the file is damaged or its data uses an unsupported feature. Animated inputs become still images. Try a current browser or obtain a fresh copy of the original.