Why convert JPG to PNG
Some apps, games, printers, design tools and upload forms ask specifically for PNG. PNG is also the better working format when you will edit a picture many times: every time a JPG is saved it loses a little quality, while a PNG can be saved again and again without any further loss.
What happens when you convert
The JPG is opened in your browser and saved as a PNG with exactly the same width, height and colours. Nothing is cropped or resized. Several JPG files can be converted at once and downloaded together.
What converting cannot do
Converting does not improve the picture: blur, noise or blocky marks already in the JPG stay in the PNG. It also does not remove the background — a JPG has no transparency, so the PNG has the same solid background. And because PNG is lossless, the new file is usually several times larger than the JPG.
Common problems and how to fix them
The PNG is too large for an upload: PNG is not the right format for big photos — use the original JPG or Compress Image. You wanted a transparent background: you need a background-removal editor; this tool does not cut out objects. Colours look slightly different in another app: some programs show colour profiles differently; the pixel values themselves are unchanged.
Good habits
Convert to PNG at the start of an editing project, keep JPG for sharing finished photos, and store your original camera files separately.