I normally don't use the whole image as a thumbnail for any of several reasons.  For starters, I may not want to give away the whole scene.  If you've posted the whole thing in mini form, that leaves little reason to click to view the whole thing for many people, or perhaps the subject of the image is unclear in such reduced form.   Next there is the issue that I often like to try to put something that draws the eye or something unique to the image in my thumbnails.  Finally, there are the times when I want the thumbnail to highlight the detail in some portion of the image if I feel that's an especially important point.

I am, however, quite against the practice that some people use of deliberately misleading thumbnails.  I really hate those >.<.

¤~Giolon~¤

¤~ RadiantCG ~¤~ My Renderosity Gallery ~¤