MizzBia - 1:56 am on Oct 12, 2008 (gmt 0)
Two things: Creativity and Formal Training.
It helps if you are creative to begin with but I guess anyone can learn by mastering the tools needed to accomplish your task. In this case, it would be Photoshop, Fireworks or even Illustrator. If you do not plan on going to school for this, I recommend picking up some books on the subject and then practice as much as you can. The more comfortable you are with Photoshop, the easier it will be to turn your ideas into a realization.