Has anyone heard of GIMP? Basically, it's a popular photo and art editing software which, like WordPress is for blogs, is totally free - but not sure if open source. Anyway, I've tried it myself and you can do a lot - assuming, as with Photoshop, your willing to study. Anyway, while we're on the topic, another popular free art software is Blender. Basically, it's a 3D modeling software and like GIMP is probably tough to learn, but definitely worth the time.