I had a friend who was doing a lot of this and he said roughly the same that some unusual photos sell the best and its not really obvious at the start what those type of pictures are. One good idea is to look at news sites and see what photos they attach to news stories. My mate was taking crazy stuff like a pair of football boots propped up with the goal net in the background and pictures of garbage bins etc. I also know that the most popular 3D object sold on the big repositories are baseballs and footballs - i.e. spheres ! They are used to make those cheap animation cuts between shots on scoreboards.