It seems that because I use my Sims images for the relationship tab as well as the family trees that they are exceed the "10 times used" limit, and therefore excluded as being used for category listings. I do that so that I don't have to update multiple files whenever a Sim has a birthday.
I have tried to change to using an age-specific image that isn't used so heavily here. I know that it takes time for the cache system to catch up with the change, but I would like to know if I am right in that this is what is needed to make it so that the images will actually show up in the listings. If it does work, it will be a little of a hassle to have to change images for each birthday, but I am willing to do it so that they show in the listing.