Open the file up via Paint. Then go to Attributes. Depending on your Paint program, you might be able to have the option of "Transparent Background." If you have it, click on it and change it to white.
White and Black are the two colors that it'll let you use no prob. Having a COLORED background is the tricky part... I have no idea if it'll work on bitmaps, though. It may only work on gifs.
Anyway, if your Paint program DOESN'T have that option, then I can't help you. Unless...you have Paint Shop Pro... *sigh*
The comp (new) I'm on doesn't have that option, but our old one does.
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Deep inside us all is a little green elf telling us to burn things.