Personally, I like Adobe Photoshop
It's a little expensive, but it will have everything you need and more.
If you use Dreamweaver, try Macromedia's Fireworks
The cool thing about Macromedia's programs is that they all integrate with each other. (Dreamweaver, Fireworks, Flash, etc...) You can get Macromedia's Studio MX that has all of the programs bundeled together.
If cost is a factor, you may want to look into JASC's Paint Shop Pro
I think it's about $100 (last time I checked) and it would be good for simple stuff like buttons, backgrounds, etc... I think it has some pretty advanced features as well, but I've never really dealt with it so I can't say for sure.
Hope This Helps!
Ecobb
"Alright Brain, you don't like me, and I don't like you. But lets just do this, and I can get back to killing you with beer."
- Homer Simpson