It only appears on the images which have been forced to display smaller than they really are. It's supposed to help indicate the image is clickable (to view the full size).
Example...
Image #1 is 70 pixels wide x 50 pixels high.
Image #2 is 480 pixels wide x 348 pixels high. Because it's width is greater than 350 pixels, there's a javascript function which tells the browser to display it as 350 x 254 instead of it's original dimensions. Although the dimensions within the browser has been changed, the actual file size has not changed.
Did I lose anyone -- show of hands if you're still confused.
![Wink [;)]](/FSM/CS/emoticons/icon_smile_wink.gif)