I was hoping you could clarify for me the process for mounting multiple images per slide. Is this possible through templates or drag-and-drop within the program, or is it necessary to create a composite outside the program (LR or PS) and import?
You can drag-and-drop from within the program 
Go into Storyboard or Timeline mode and drag an image onto a slide from your image list (on the right). Now drag ANOTHER image from the list to the same slide and place it “above” the previous image (I think you see a yellow highlite). It is now “layered” above the previous image. If you want to change the layering, just click on the image and move it up or down in the stack. Click on each image you want to work with (one at a time) to change its size, rotation, or placement. Each image can have a mask as well. The maximum number of images per slide is 6 (I think that’s still the limit, anyway). Or, you can combine images AND text – still, the limit is 6 stacked items.
That’s kind of unfortunate – I wish we could place as many images AND text blocks as we wanted, but we can’t. I’m guessing it has to do with how fast FM can read in those images and display them “on the fly” when it comes to a presentation (too many and it probably slows too much). A GOOD slide show SHOULDN’T use that many images per slide anyway, but still it would be better if we, the users, had a choice.