Okay.. :) Welcome here also :D
As you probably can see, almost all game files are .bnk's. If you open a .bnk file (with the bnk unpacker) you find lots of .2db's.
These 2db's you need to convert into .dds's with TDU Modding Tools. :) After you done that, you can open them up in any other piccy program, like Photshop and edit them. This technique is useful for smaller modding, like head/taillights, or making your own numberplate, stuff like that. ;) When you are done with the texture, you have to do it backwards, convert the edited file back into a .2db, and then pack that into the original .bnk. :)
I hope I made sense there, and didn't forget anything ;) If I didn't make sense, tell me ;)
About the menus in game. The only way I know of would be to change that inside the game. Should be under Options somewhere ;)