PNG to JPG: what happens to transparency?

JPG has no alpha channel

The JPG format stores only color information, not transparency. When a PNG with transparent areas is converted to JPG, every transparent pixel is filled with a solid background color.

The converter uses white by default, which is the safest choice for most documents and photos. If your image needs a different background, flatten it onto that color first.

When the fill color matters

A logo with rounded corners and a transparent background will get white corners in JPG. On a dark website those white corners will be visible, so keep the PNG version for the site and use JPG only for light backgrounds.

For photographs the transparent areas are usually invisible anyway, which is why photo-to-JPG conversion is so common.

Check before you replace the original

Compare the converted JPG against the PNG at full size. Look at the edges where transparency used to be, and confirm the colors look right. Keep the PNG as the master file whenever you may need transparency again.