Perhaps I did'nt understand right, but I think he wants to use a 2D-Character as a 2D - Object in a 3D World, not that he wants to generate a 3D character based on a 2D image.

So, it's pretty hard to do it well, but you can goal with it. If you wanna use 2D Sprites, you must save 24 images of motion for one second to get a good (should I say recommend?) animation-cycle. For example: you want the character goes straight ahead, and the cycle is a half second, you must create 12 frames. But that will produce a lot of diskspace, man. Do ya want to create a cartoon-style game? I would prefer in this case 3D Models with faking-shape and cartoon-art. But if you can't model and want to search for another solution, ask someone, if he helps you.