I would then suggest adding in this ability to the plugin. The only place it would really be a problem is on the homepage, and archive/taxonomy pages.
For instance:
I use the wpml (multilingual) plugin and have it setup so that each language actually uses its own domain name. When I create a new page in English, I set the banner. I then go into the system, and duplicate the page for Spanish. This is no problem because I can see your banner image select box just fine as technically I am creating a new page even though it is in a different language.
Obviously this doesn’t work for the archive/taxonomy pages as you don’t actually create a new page for these for each language.
Well it is just a suggestion, as it would make it much possible to use your plugin on multilingual websites, where right now I don’t see how it is possible.