Sometimes when that happens to me, I just right click inside the small box and chose to open in a new tab or window. Then if that image does not display, you can hit the refresh button on that page with just the image open and it will reload just the image. Sometime I have to do it several times before the image finally opens. I suspect that there are at least 2 reasons for this. One, if the image is in the process of being updated, then the server it is on may not send out a partial image, or 2, if demand is high for a particular image, then the server may just deny the request for the image because it is too busy.
-John