I do not use Organizer, so may be missing something, but I would guess that one could Sort By, and choose File Type. Remember, Organizer is for the system, and does not reflect what is Imported for use in a Project - two different things. In lieu of Organizer, I use Adobe Bridge, installed with some of my other Adobe programs.
I would also assume that Organizer does things via folders. If one does not have any aspect set to auto, I would think that it would ONLY do thumbnails for certain folders, as directed. If one has their stills in other folders, then only AV Assets would be seen by Organizer.
Maybe some Organizer guru will have better directions for you. I am just thinking here and speculating.
If you go to the Organizer's View menu, you'll see the options for displaying only videos, only photos, only audio, etc.
I cover the Elements Organizer and it's many functions in my books at Amazon.com and at the Muvipix store, by the way, if you're interested.
Thank you for that clarification. I assumed that one could Sort By, and you have pointed out an even better solution - View.