GIMP, but I start with a transparent background. If you want to add transparency to an image you'll have to either erase manually, or tell the thing to make some certain colour transparent (like with a greenscreen)
Yes, if you go [colour] and select [colour to alpha], whatever colour you select, turns to transparency. Although since the colours are mixed, this has a nasty way of making other parts of the pic semi-transparent. That's why erasing is easier, but a nightmare when it comes to anything not pixel (because it takes forever and is hard to do perfectly).
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