Also, check out Krita; it looks a lot like photoshop to me, but the last time I used photoshop was 7 years ago. https://lh4.googleusercontent.com/-dBDHsjsOG4w/U1e8ubOrBTI/A...

There is a guy, David Revoy, that uses only free software to do it's job, and it seems successful at it. http://davidrevoy.com/