GIMP and Paint.NET are free (but I don't use it :P). They are like mini-photoshop. I'd use it if I had a tablet, but I don't. Hope I get one for Christmas!
Woah, gigantic signature. Fyi, people are more interested in the picture than your name, y'know? If you ever paint or draw for real, having a signature that big will immediately make the piece worthless, unless you're Michelangelo =/