Photoshop is the old standby, but I don’t like it for video graphics. Someone as simple as grabbing an element and moving it is problematic. I much prefer FireWorks, or even Motion for still graphics, which has some nice layout functions.
GIMP is a free, open source graphics program very similar to Photoshop. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. It works on many operating systems, in many languages.