There's always pixlr.com :) Though it isn't as good as GIMP, it's the best one on the website that I know of :)

I don't know any websites, but I do know a free program that you can use. Have you ever heard of GIMP? It's basically like... a more simplistic version of photoshop that's a very good starting program for beginners. The interface is easy to figure use (even with no prior experience), and did I mention it's free? Haha.

You can find it there: http://www.gimp.org/

i used www.postermywall.com