*poke*
You need a program called HMT (Halo mapping tools)
Google it, and download.
Then with that you open up a map. C/: > Program etc.... Halo> maps > blood gulch (just as example.)
Then in the program opens all the bits and bobs for the maps. Go to the 5th one down (i think) called Bitmaps (234) < should have a large number, can't remember what. Then scroll down to find vehicles/warthog/vehicles/warthog < i think. Click it, and on the right will come up the default hog skin. Go to halomods.com and sign up to the forums, then under the board Halo PC is a sub board called Downloads. Search the threads for hog skins, download them. Then go back to your HMT and below the pic of the current skin is a button called inject texture. Click it, then search/open the skin you downloaded, and it replaces it for all maps. Some skins have rims and a turret bit, so they go with vehicles/warthog/vehicles/tires or something like that.