That's a thumbnail linking to a page. If the link says
<a href = "/images/greatbigpicture.jpg" target = "_blank">
<img src = blahblah...></a>
then once you're on the great big picture, you can't go anywhere.
I now realize that I've noticed the same phenomenon as Mike: big picture in Image Search, combined with the gallery page (including all thumbnails) it belongs to. That is, for Image Search purposes there doesn't seem to be any difference between <a href> and <img src>. In logs it tends to come through as two copies of the gallery page, with the blowup sandwiched in between. Not sure what user behavior this reflects; it's not an artifact of the search itself, because there are several seconds between.
Some robots definitely make the distinction, though. I've seen a good number of those single-attack blitzes where they follow every <a href> regardless of what's at the other end.