GIMP :: Merging Two Images And Cropping Anything Outside Of Certain Area?

So I'm trying to make a modified loading screen for a game (Homeworld 2 to be exact) but I'm not sure how to do this. You see the loading screen has a black background with a blue line in the shape of a hud, with the actual picture inside of it (Example, the loading bar is rendered dynamically and not part of the image). What I was thinking of doing, is erasing the inside, then copy & pasting the picture I want as a new layer over it, then cropping out the area outside the blue "hud" to give a black background with a internal picture. I was wondering if there would be a way to do that without overwriting the blue outline and having to do it manually.