I would say Paint.NET. Why? It's free, it's powerful (AFAIK), and it's easy to use. I've seen Photoshop, tried it at school, but I can't really figure it out. There's also something like Corel Draw, I forget it's name, but I tried that out at school too, and I found it was fairly easy to use and pretty powerful. But my vote still goes to Paint.NET.