I think the choice of thumbnail depends on the subject matter. It also seems to matter if there is a female in the picture or not. Most pictures with thumbnail manage to show some part of the female (face, boobs, etc). I don't consider showing a part of the image a deception, as much as a tease. The thumbnail should reflect the picture quality. Whether I use the whole pic or the thumb depends on the image. If showing the whole pic is the only way to do it justice, then I show the whole pic. If there is a thumbnail that summarizes the entire pic's intent and quality, than I use that, especially if the picture is too detailed to just look like a blur as a thumbnail. While I have been guilty of a couple tease thumbs, I generally follow the rule above.

The name of the image also has a lot to do with the number of views. "Girl on Beach" may not get the same response as "Hot Woman & Summer Nights." Of course, like the thumb, which one entices more is dependant on the viewer. Swannie's "Hot P**sy" in the photography section is a good example of the name enticing the viewer...

Just my 2c