The format has no alpha channel
PNG stores four values per pixel: red, green, blue and alpha, where alpha says how opaque the pixel is. JPG stores only three. There is simply nowhere to keep the transparency information.
Why you sometimes get black
Transparent pixels often carry the colour values black underneath. If a converter drops the alpha channel without compositing, those hidden colours appear, which is why some tools turn transparent backgrounds black. Good converters first paint the image over a chosen background colour. This site does that and lets you pick the colour.
Choosing the background
- White works for documents and most web pages.
- Match the background colour of the page or slide where the image will appear.
- Soft shadows and semi-transparent edges blend into whichever colour you pick, so choose the real destination colour for clean edges.
If you need transparency
Keep the PNG, or use WebP or AVIF, both of which support alpha and are usually smaller than PNG.