The one detail this tool gets right on purpose
JPG cannot represent transparency at all. If a naive conversion simply drew a transparent PNG onto a canvas and exported it as JPG, the transparent areas would render as solid black — not white, not any sensible default — because that's the canvas's default empty-pixel value, and JPG export has no concept of "leave this transparent." This tool avoids that by explicitly filling the canvas with your chosen background color (white by default, confirmed here: white) before drawing your image on top.
What stays the same
Dimensions are preserved exactly — this tool changes format and fills transparent areas, nothing else. The quality slider works the same way it does for any JPG export: lower values trade visual detail for a smaller file.
Choosing a background color
The default (white) works well for most images headed for a plain background. If your image will sit on a colored page or template, matching that color here avoids a visible white box appearing where the transparency used to be.
Privacy
Your image never leaves your browser. Nothing is uploaded, logged, or stored anywhere.