I would suggest adding the ability to have User add any folder he/she wants to be used for slideshows. It could just be a link to User's "family photos" or "artist art" or whatever. So User would go to "Theater View>Theme>Online Media slideshow" and there would be an "Add" button with a browse to folder capability.
If that is too difficult (and it sounds hard) perhaps just add an empty folder (i.e. "Slideshow Images folder") to the existing list (the Google, Last.fm, HTbackdrops, etc.) and then User can put any "jpgs" in that folder they want to see in their slideshow. The actual location of this Slideshow Images folder would just be next to existing Cover Art, Artist Art folders.
To add: I don't know how others are think of using this feature, if it is added, but I would just want to have a bunch of my pre-selected photos (could be family holiday or vacation or wildlife photos) randomly mixed in along with whatever comes from the other sources I have selected (i.e. last.fm and Google). The "Slide Show Images" would not be linked to any particular song/album. So if I had 100 images in the folder, they would all show at some time during the slide show (if the show plays long enough).
To add2: Thinking about this more, it would be better if the User could add multiple folders to the list (like a Holiday Images, Music-Themed images, etc.) then one could select the folder that he/she want to be included in the Slide show by checking or unchecking the box. So if I have my Holiday Music Playlist running I could check the box for the Holiday images, and only have those showing.