You need to submit it as a normal image and not a file containing an image, that way people would be able to use it on dA and what not. The manner you have it now it seems like they'd need to download your file, unzip it and reupload to their own dA account in order to be able to use it like a normal stamp, which is really rather troublesome when you could have submitted just the stamp without the grey background.
Like here, when I tried to use your stamp like I'd use any other stamp, but I can't because it has this grey background and makes me not want to use it like I use my other stamps like the one on the right: [link]
I'm just trying to figure out why you have to use those grey backgrounds to begin with, for any of your arts, especially for stamps. Unless you don't want them used on dA? I just find that odd.