What JPG to PNG Actually Does
JPEG compression throws pixel detail away every time a file is saved. PNG never does. Converting doesn't restore detail the JPG already lost, nothing can, but it stops the bleeding: from this point on the image can be opened, edited, and saved as often as you like with zero further damage.
That makes PNG the right container for anything headed into an editor. Crop a JPG, save it as JPG, crop again, save again, and the artifacts stack. Do the same round trips through PNG and the last save looks identical to the first.
When the trade is worth it
| Situation | Convert? |
|---|---|
| Screenshot with text you'll annotate | Yes, text stays sharp through edits |
| Photo you'll edit several times | Yes, until the final export |
| Photo going straight onto a web page | No, it'll just be 5 to 10 times heavier |
Expect the PNG to be noticeably larger than the JPG it came from. That's the price of lossless storage on photographic content, and the byte readout above will show it plainly. For the web, run the finished edit through PNG to WebP and get the weight back.
The reverse direction
Going PNG to JPG is a job for the main converter: pick JPG, set the quality slider, and know that transparency will be flattened onto white, since JPEG has no alpha channel.