For people who design graphics and do digital imagery there is a couple of programs i would suggest that I use. Firstly would be Photoshop because it is a very advanced program great for everything including animations and secondly Gimp basically a free version of Photoshop with different aspects of design.
For hand drawn art to a comic book look through the computer I suggest Adobe Illustrator it is great when you use live trace to give the comic book cartoon look.
Hope this has helped.