You'll need a photo editing program. PhotoShop is the king, and you'll be able to do just about anything you can imagine with it (it also comes in handy to create masks, graphics, etc. for video...) Jasc PaintShop Pro is supposed to be pretty good (and much cheaper).
Of course Photoshop is king and the latest version rocks, but very expensive. Paint Shop Pro is very good, GIMP a clone of Photoshop is excellent and totally free.
For adjusting color levels, salturation, color correction, stuff like that Vegas has many built in filters.