This might help you. If you haven't already looked through it.
https://www.townlong-yak.com/framexm...ainerFrame.xml
If you were talking about alternative images, you might find these useful. The ContainerFrame folder will have the image files used by default.
https://github.com/Gethe/wow-ui-textures
If you are looking for alternative images that have already been used, maybe look for similar addons and see what they use.
If it's none of the above, hopefully someone can offer some pointers. Good Luck.