What happens to transparency?

It's replaced by the background colour you pick, because JPG can't store transparency. The option only appears when one of your PNGs needs it.

When should I keep PNG instead?

Keep PNG for logos, screenshots and anything with sharp edges or transparency. JPG suits photos, and it can blur text.

It depends on the picture. Photos often shrink 5 to 10 times. Flat graphics can barely change, or even grow.

No. Converting happens in your browser with WebAssembly, so nothing is sent to a server.

PNG to JPG

Turn large PNGs into much smaller JPGs, with control over transparency.

Runs on your device. Nothing is uploaded.