It's like photoshop - hard(ish) to learn properly, but once you do, it's a piece of piss
I think photoshop and GIMP do it.
( Methylene Blue - electrohead, Sat 30 May 2009, 16:26, archived)

cheers.
i'll investigate.
( tickle I was away, Sat 30 May 2009, 16:41, archived)