Firefox: On mouseover, the cursor becomes a magnifying glass. Click to display at 100% size.
Internet Explorer: Mouse over it.
Wait for that little box of buttons (save, print, email, My Pictures) to appear in the upper right hand corner.
Wait some more, for the "zoom out" button to appear at bottom right. Click on it. Except BE CAREFUL to keep the cursor inside the edges of the picture on your way to clicking it, or else all the buttons will disappear and you'll have to start all over, doubling your wait time.
Granted, the IE way of doing it is not exactly a Dickensian hardship. But still, compared to the instant convenience of the Firefox way, the IE way just seems DUMB.
I wonder how other browsers handle it...