When to choose PNG instead of JPG
Pick PNG when the image has transparent areas, when it contains text, screenshots or line art, or when you plan to keep editing it. PNG is lossless, so converting from JXL to PNG adds no new compression artifacts.
The trade-off is size. A PNG of a photo is often several times larger than the original JPEG XL file, and larger than a JPG. For photos you only want to share, the JXL to JPG converter usually makes more sense.
Private by design
The JPEG XL decoder runs inside your browser using WebAssembly. Your images are not sent to any server, and the tool keeps working if you go offline after the page has loaded.
Questions
Is the PNG an exact copy of the JXL image?
For standard 8-bit images, yes: PNG adds no compression of its own. Images with HDR or more than 8 bits per channel are converted to standard 8-bit color.
Is transparency kept?
Yes. Transparent and semi-transparent areas stay exactly as they are.
Why is the PNG so much bigger?
JPEG XL compresses images far more efficiently than PNG. The extra size is the price of a format that every app can open.
Is it free, and are my files uploaded?
It is free, with no sign-up or watermark, and nothing is uploaded. The decoder runs in your browser.