Yuletide is coming!

I have started to see people's Yuletide icons appearing (...posting about Yuletide probably helped with that) and, as it does every year, it was making me sad that I didn't have one. So, hey! I could ask!

Could anybody put a (wiiiiildly inappropriate, yes) Santa hat on the icon below, or tell me how to do it myself on a Windows 7 PC sans Photoshop in very very small words?