Does Converting JPG to PNG Improve Image Quality?
Converting a JPG to PNG does not repair blur, compression blocks, or detail that the JPG format already discarded. It can still be useful: the resulting PNG is a lossless working copy of the pixels that remain, which helps prevent additional compression during later edits.
What the conversion does
A JPG is decoded into pixels and those pixels are written into a PNG file. PNG uses lossless compression, so it preserves that decoded image during future saves without adding new JPG compression artifacts.
This can be useful before you annotate an image, add layers in a design tool, or submit a file to a system that accepts PNG only.
What it cannot restore
The conversion cannot recreate detail that was removed when the JPG was first compressed. It also cannot remove a baked-in background or create transparent pixels, because JPG files do not contain transparency information.
If you need a transparent subject, use a background-removal or image-editing workflow after converting. Changing the file extension alone is not enough.
Why the PNG may be bigger
Photos usually compress efficiently as JPG. PNG is often much larger for the same photo because it preserves pixel data without the kind of lossy photo compression JPG uses.
For a web photo where file size matters, JPG or WebP may be the better final format. Use PNG when its editing or compatibility benefits matter more than file size.