FWIW, set my IE7 exactly as yours and tried it:
  1. Went to http://www.google.com, left-clicked the 'Images' link at the top left and it loaded http://images.google.com/imghp?tab=wi ... in the same tab.

  2. Left-clicked IE7's Back button to take me back to http://www.google.com

  3. Clicked the same 'Images' link using the 'mouse scroll wheel' this time and IE7 opened a new tab with the http://images.google.com/imghp?tab=wi page in it.
Also tried the same test with Firefox 2.0.0.6 and got the exact same results.