You can use basicly almost any kind of graphic or photo editing program.... Photoshop, Paint Shop Pro, Microsoft Picture It!, PhotoImpact, the Gimp, even MS Paint... to name a few.
There are several, some a lot more expensive than others. Photoshop (expensive) seems to be the most popular. There's a "stripped-down" version, Photoshop Elements, and also Paint Shop Pro, which are about the same price. There are also some free programs, like The Gimp, which is said to be very complicated, and Photofiltre. If you browse through this forum, or do a search using the names, you'll find info and links.
This thread might help:http://www.thecatsite.com/forums/showthread.php?t=76789
For a free program that's easy to figure out, I highly recommend Photofiltre
It may not seem like it can do much, but explore it and figure out tricks & things ... I use it a lot and have found it can do a lot more than you think!