Software can really be a personal choice
when it comes to digital artwork, but
there are some common ones out
there used by many.
Paint Shop Pro
Used to be owned by Jasc. Now owned by Corel. There are advantages and disadvantages to both. For quick and dirty resizing, etc. I use PSP 7 created by Jasc. But when I need some extra features like the Mesh Warp Tool, PSP X by Corel fits the bill. Just be ready as it is a memory hog. It's still cheaper than Photoshop if you're a little tight in the wallet.
Photoshop
The standard in all digital graphics. Can do some pretty amazing stuff.
Gimp
The best price of all! FREE! I've never tried it, but have heard good reviews.