How this works
This re-encodes the image as PNG using a lossless, optimized encoder (adaptive row filtering plus DEFLATE compression) — since JPG doesn't have transparency, the output PNG is fully opaque; converting doesn't add an alpha channel that wasn't there.
Common questions
Does converting JPG to PNG improve quality?
No — JPG compression is already lossy, so whatever detail was lost when the JPG was created stays lost. PNG just stores those same pixels losslessly from that point forward, without adding further loss, but it can't recover already-discarded detail.
Why is the PNG file bigger than the original JPG?
PNG is a lossless format and JPG is lossy — lossless encoding of a real photo (lots of colour variation) typically produces a larger file than JPG's compression, which is normal, not a bug.