Graphics modding consists of creating custom models and/or textures for the objects in the game.
Making your own Models
We generally suggest using 3D Studio Max or Maya. There may be other things that can create .X files. Hardpoints are NULL points on the model.
Here's an example: http://forums.galciv2.com/?ForumID=348&AID=103510
What determines whether your creation is a hull or an extra is how it's handled in the gc2types.xml file.
More to come here. The files are in the \galciv2\gfx\models directory.
Before distributing mods, wait until Stardock has documented the \MODS directory so that your mods don't get overwritten. You don't want mods you give to others modifying the gc2types.xml file. The Mods directory will go by the keyname, not the file name.