Greb, if you want to use your usual "Pictures" folder in your library or any other you like you could do a symbolic link since windows vista
Here is how to use it: http://technet.microsoft.com/en-us/library/cc753194%28WS.10%29.aspx
It works perfect. Only problem is that everytime you want to change the folder you must enter command prompt, go to musicbee folder in dos and create a new symbolic link...
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Steven, the idea is giving the option to select the background images folder.
Imagine you're having landscapes as theater modes backgrounds. But you want to use a different folder with, let's say, animals, or cars, or milla jovovich pictures

Right now you must change the theater mode file in order to aim the new folder. And besides that, all your folder with images/wallpapers must be in the Program Files\MusicBee\Plugins\TheaterModes\SharedArtwork folder, that is not a very easy path if you want to use those wallpapers for any other thing (for example, as wallpapers for your desktop).