I figured it out after about 15 seconds from seeing one of the images load on-demand. Which is the time any self-respecting computer programmer should take.
I load links as background tabs and visit them later when I am on a slow connection. When sites do things like this I end up viewing the site for the first time several minutes after clicking on it only to find that I had been waiting for nothing.
Right, I'm in a position such as yours every now and then. Now I understand that you meant that you can't possibly know you have to force the images to load because you have not looked at the tab yet.