The image you use has to be 255K or less, and in jpeg format. There's a gallery to browse, too.
Of course, you'll have to add "real" postage to the envelope. But if you don't have time to watercolour a set like mine, by all means don't let that stop you from having and using original postzegels. Why should the post office have all the fun? If you do watercolour your own, you can colour photocopy them onto a sheet of sticky back paper.