JPG to PNG: what actually changes
The honest answer on quality
JPG is a lossy format, standardised in 1992. Each time it is saved it discards detail it judges you will not notice. PNG, introduced in 1996 as a patent-free alternative to GIF, keeps every pixel exactly. Converting JPG to PNG therefore stops further loss, but it does not rebuild what the first save removed.
Why designers still ask for PNG
Design software, mock-ups and slide decks behave better with PNG, because you can crop, annotate and re-export without the slow blur that repeated JPG saves cause. Screenshots with text and thin lines also stay crisper in PNG.
What to expect from file size
Expect a jump. A 2 MB phone photo can become a PNG several times that size, because lossless compression handles photographic noise poorly. For a single logo or screenshot this is fine, but converting a hundred holiday photos will fill a folder quickly.
Transparency does not appear on its own
People often convert to PNG hoping to remove a white background. The conversion keeps the background as solid pixels. Remove the background in an editor first, then export as PNG to get real transparency.
When to leave it as a JPG
Photos that will only be viewed, emailed or posted to a website are usually better as JPG. If an upload form accepts both, keep the smaller file.
Plan to edit the image several times? Convert to PNG once at the start and save the final version as JPG at the end.