But only in terrain you build yourself w/ terrain editor.
Nope, you can load nose art in any terrain, though it is squad specific.
I think you have confused the issue of loading your own artwork, which can only be done in your own terrain. You can actually change your aircraft appearance with your own textures and art IF you put it in your own terrain. You can't change it for other terrains you didn't create (like the main online arenas). Lots of CT and Scenario setups have had their own artwork.
-Soda
Aces High Trainer Corps
The Assassins.