There is a great open source art program called Tuxpaint, it's aimed at kids. That doesn't really matter though, because it's fun for everybody. Anyway, the program has a great collection of stamps to use, and there is a collection of planet stamps, but no Pluto. Even though Pluto has been demoted from "planet" status (Wasn't it always an Ice Dwarf?) I still thought I would donate a nifty Pluto stamp. I'm posting it at devART first so I can release it with a CC license.