HoverZoom is a neat little extension I use on Chrome. Thumbnail Zoom Plus is a great alternative to this if you are using Firefox. It has all the same features and works on the same websites. Just hover the mouse on an image to see the fullsize version of it, if it exists.
Settings I use with this extension:
Set the Delay before display to 0.5 sec.
Set Default zoom to 100 percent.
Disable Show picture using all available space.
Turn off all keyboard shortcuts.
Tried with: Thumbnail Zoom Plus 2.7, Firefox 29.0 and Ubuntu 14.04