Yeah, save it as either a tga, or use a free program like VTFEdit (http://nemesis.thewavelength.net/index.php?c=178#p178) to turn it into a VTF file. If you use vtf edit, however, make sure that when you are importing the image, uncheck all the boxes on the window titled VTF Options. Otherwise you will have normal maps, mip maps, all that stuff thats unnecessary for a simple spray image. Good luck!