Choose for the image and destination, not the extension alone
PNG uses lossless compression for its stored pixel data and can include transparency. JPEG uses lossy compression and is commonly suitable for photographic content. PNG is often a better choice for small text, screenshots, and flat graphics, while JPEG can be useful for photographs when file size and compatibility matter.
Those are starting points, not a promise that one format will always be smaller. Dimensions, image detail, transparency, and encoding settings all affect the result. Compare actual files at the display size you need.
A transparent PNG needs a background when it becomes JPG
JPEG does not store an alpha channel. The PNG-to-JPG converter combines transparent areas with white before encoding. A logo intended for a dark background may therefore look very different after conversion, especially around partially transparent edges.
Converting that JPEG back to PNG will not recreate the original transparent pixels. Keep the original PNG, or prepare the intended background in an editor before creating the JPEG if white is not appropriate.
Maximum JPEG quality is still lossy
The PNG-to-JPG interface uses maximum JPEG quality, but quality 100 is not a guarantee of unchanged pixels or a small file. Inspect fine lettering, high-contrast boundaries, and subtle color transitions. A format conversion is not the same as a target-size compression tool.
Repeated JPEG editing and saving can add compression changes. When a workflow requires PNG, converting once to PNG avoids another JPEG encoding stage for that copy, but it cannot undo damage already present in the source.
Converting JPG to PNG does not restore detail
A PNG made from a JPEG stores the decoded pixels losslessly. Those pixels already contain any blur or blocking introduced by the original JPEG. The larger output that sometimes results is not evidence that new detail has been recovered.
- Keep PNG or SVG when transparency or crisp flat artwork is important.
- Use JPG when the destination requires it and a white matte or opaque image is acceptable.
- Use JPG-to-PNG for compatibility or an editing workflow, not automatic restoration.
- Compare output appearance and bytes before choosing which version to distribute.