Ok, so a search of the forums shows a few people have been requesting something like this off and on all the way back to MC21...
Currently, Online media slideshow allows us to pull background images from Google, last.fm, and the TheMovieDB seemingly at random. The problem is many times the images that pop up are inappropriate or just the wrong thing (e.g. Disney's animated Beauty and the Beast vs. live action are mixed up).
Please please please add an option that lets the users set up a hard drive folder or folders with some kind of naming convention that will match the media we are viewing in Theatre View with one or more jpg files that will show up in the background slide show just like the Google image search works right now. I could then pull images I like best (or make my own) and store them locally and have MC pull those images when displaying the slide show.
I've been trying to find a way to get this to work using custom themes, but I don't have the technical wizardry to figure out how to set up my own and I LIKE the way default Theatre View looks except for the tiny problem of media slide show giving me incorrect images.