Converting to PNG is the right move for some images and pointless for others. It is worth knowing which you have.
PNG cannot reverse JPG artefacts or make its background transparent by conversion alone. The PNG can be larger. Background removal is a separate task.
Transparent areas become white in JPG. PNG preserves existing transparency but does not remove a JPG's background.
PNG ignores the quality slider. JPG and WebP encoding can change detail; this tool has no lossless-WebP switch.
A PNG is useful when you are going to edit the picture again, because saving a PNG repeatedly loses nothing, while a JPG gets a little worse each time. It is also the right format for a screenshot or a graphic that you want to keep sharp from now on.
Converting does not undo the damage a JPG has already taken. The blocky look around edges stays. The file will also grow, often by several times for a photograph, because PNG stores every pixel exactly.
No. Changing encoding cannot reverse earlier compression or create missing pixels.
JPG, PNG and WebP always open, and other types depend on your browser. A very large photo can run out of memory on a phone, so try a computer.
PNG is lossless and is not designed for photographs, so it stores far more data. That is the price of keeping every pixel exactly.