There are several free image editing tools : IrfanView, Paint.NET or (a simpler and not so well know ) BS Picture. Depends on your needs which one you want to use.
I have to a gree with bobelinho, but if you want a photoshop like program I would recommend the GIMP. It will take a little learning but you will love it in the end.
GIMP is good once you master it, but there is a bit of a learning curve involved. Haven't tried Paint.NET. I think they were implying that u cn get a cracked version of photoshop relatively easy.
Artweaver 1.5 is now released. Artweaver is a painting program which is suitable for beginners but also suitable for advanced users. You can record all your steps and replay them afterwards or provide them to other users.