How is this creative, Rantent??
Anyway, I don't know how to make it so SC saves them in .bmp format but you can open and change format with this free program.
http://www.staredit.net/index.php?download=509 Also, .bmp isn't the best format to choose. Filesize is just huge and a lot of websites doesn't accept .bmp files. I say try .png, it has the same quality but is way more compact.