I've had fair success using Gimp for similar use-cases -- with the caveat that that is mostly for design mock-ups made in photoshop (no vector graphics to speak of).

There's definitely a learning curve, and Gimp still has a long way to go -- but it's passed into "usable" quite a while ago for my uses.

[edit: if anyone knows of other alternatives (preferably that runs on Linux and/or under Wine, preferably Free software - but also closed source -- I'd love to hear about it]