The transparencies used in the Adventurer Avatars pack of circular tokens I created were accomplished by creating an image on a single layer without a background layer in Photoshop and then saving it in a PNG format. Unlike JPGs the PNG format can support transparencies. I haven't used GIMP but I'd be shocked if it couldn't do the same thing. That being said, Token Tool and various other token creation programs out there are arguably much faster and easier to use if you are unfamiliar with image editing software.