There are several options for graphics programs. The very top of the list is Photoshop, which does everything you can imagine, but it's quite expensive. In the more affordable price ranges there are Photoshop Elements (which has fewer features than the full package) and PaintShop Pro, which is quite full featured, but has it's own interface style. And to get started with NO money, there is
GIMP... a freeware graphics application that is fully capable compared to Photoshop Elements and PaintShop Pro, but lacks a few of the very top end features of the full Photoshop package.
Myself, I've used Photoshop Elements for years now, and am currently experimenting with the new version of GIMP.