A thumbnail should probably be a reduced version of the full image. However, it seems that a good thumbnail is becoming an artform in itself. It's true that if you can see the whole image in a thumb, there's not much point to clicking on it to see the full version. Sometimes though, the thumb image is too tiny to make out details and it's worth it to see the big picture.

By creating a "teaser" thumb, people attempt to attract viewers by tantalizing them with a well placed snapshot. For those of us that like (and are able) to view all images, this has had an annoying tendency to focus on the chest of any females present in the image. I can understand what the artist is doing: Breast shots generate more hits. The other extreme seems to be taking a teeny tiny portion of the image (such as an eye) hoping curiosity will get the better of the viewer.

Do I like cropped thumbs? Sometimes, if they're artfully done. A good thumb will get me to click on it if it looks original, or well placed.

Are people likely to stop using these tactics? Doubtful. :D