Hi Antics, Now I may have misunderstood what you are trying to do and it surely wouldn't be the first time I misuderstood someone.
Here is how I change the backgrounds in my custom guis....
Find wnd_bg_dark_rock.tga and wnd_bg_light_rock.tga. Replace them with whatever you like.
For instance. I'm using a heavily modded Celtic GUI from T.King but I didn't really like the backgrounds in all the windows. I found a skull background someone made (don't have the link handy sorry) . I renamed the wnd_bg_dark_rock.tga and wnd_bg_light_rock.tga to something else (ie. wnd_bg_light_rock-default.tga) and copied the new ones into the folder.
VIOLA!
Hope that helps.
Arc