There is a version of Gimp that emulates the Photoshop environment. I haven't tried it myself and it does not include all the functionality of Gimp but it's worth a try. You can find it here:
http://plasticbugs.com/?page_id=294
It's called Gimpshop!
Look for other plug-ins and things for Gimp as well that makes the interface better.
For pure sprite-based graphics I use GraphicsGale. It's a pleasure to work with. You can find the free version here:
http://www.humanbalance.net/gale/us/
Hope this helps. Otherwise it might just be worthwhile buying Paint Shop Pro.