Since this thread is a question and not really for displaying art, I will move it over to General Discussion.
With that out of the way, the best free program out there is GIMP
http://www.gimp.org/
It is essentially the free Photoshop, almost everything that can be accomplished in photoshop can be accomplished with GIMP. Although you might want to look over a few tutorials first before you really get into it. It has a little bit of a learning curve, but you eventually get used to everything.