Why don't you modify the image paths in the existing GUI profiles rather than overwriting the images?
The only images I overwrite are the buttons, because GUIs are incredibly inconsistent. The game seems to handle most gui types in completely different ways. Modifying the image paths doesn't work for windows, and I'll have to slap myself on the face if I somehow didn't try it for buttons too. Replacing images via script will update the look of buttons, but not windows.
I'll have a second look, it's possible I missed it since the buttons were a bit of an afterthought.
Edit: nevermind, I was blanking there for a second. It is because button ui is defined for every individual button, and it is simply not practical to go through every button and change the path, and to package the function for creating new buttons. Windows, on the other hand, have the ui path defined in the profile.