Personally, I have faith in JRiver to pull this off.
The latest build of MJ does a really good job of keeping Audio, Video, and Images all seperate. From there, it's a simple task of creating 3 seperate playlist groups for the different media types, and then everything is kept nice and clean.
The playback could use a little work, as it's a little rough right now. But I'm sure that will straighten out in time.
Personally, I don't like how it will take a bunch of images as a slideshow and then if I choose to play an album, it will use those images during music playback. That kind of goes against everything we've come to expect from MJ and Playing Now.
The way I would do it is this:
If Playing Now contains images and music, then use those images as a slideshow while playing music.
If Playing Now only contains images, then only show images.
If you Play something, then wipe Playing Now and play it, just like it used to. That includes wiping Playing Now of images.
If a Video file exists in Playing Now along with music and images, play the video and then pick up on the images where you left off.
Have some way of very easily showing only music and video, only images, or all media in Playing Now that is independent of Media Library.