Rad video tools is an excellent video encoding/conversion program. A LOT of games use its proprietary video format, called Bink, including Galactic Civilizations. It will also convert graphics, sounds, etc. Extremely useful and completely free.
You don't necessarily need it to modify graphics, but you will if you want to change any of the games .bik files. If you want to edit events, techs, etc. you don't need it at all. Just follow the link that Desert Fox posted to get HawaiiFive-0's Galciv Writer. If you want to modify ships, you'll also need a program called MUtil which can be found in the mod forum.