Creating the proper thumbnail is an artform all in itself and it's something I generally agonize with a lot, even more than the artwork. When you don't usually have big breasted women in your images, it's a lot harder to pick one that grabs the audience's eyes. I usually try about 20 different thumbnails before deciding on one and even then, I've changed them out a couple times if I was wrong. Using the whole image depends on what you want to portray or the gallery you are posting in. I find that the Poser gallery seems to like the tease, and so I try to find the most attractive piece of the image, while the Vue gallery for instance likes the full image. It also depends on whether one part of an image can portray what you are trying to "sell" to the audience.

The only rule that I know of is that if there is sexy elements in the image, that is probably your thumbnail. The general audience seems to like women in their pictures and seem to gravitate toward such thumbs. In 2D, my Tale as Old as Time picture was getting buried when I had a picture of the Beast, but when I changed the thumb to Belle, it got lots more views.