First question: No. I don't think .bmps and .gifs are closely enough related for them to be interchangeable, as stated.
Michal Marcinkowski himself has told us Soldat will not have vehicles...
Mods are limited to changing graphics, sounds, text files, animations and weapon settings. You could, in theory, mess with the animations so the players look like vehicles, sorta like the LOL Mod, but then the mod will not be playable online.
EDIT: Seriously, try playing the LOL Mod online, the one with changed animations (not the original one), and tell me what error you get.