Teaser view doesn't show image field
I just tested that exact code in Drupal 7 and it's working perfectly!  The full view displays two or more images and a view of teasers just shows one image per node.

HOWEVER, I did notice the following: if you're just in the process of building a view the Preview below the view does not display the final output correctly.  The Preview in the view of teasers shows multiple images per teaser, but if you just go ahead and hit 'save' and then test your view (by going to its link if it's a page), you'll see that it actually displays correctly.

Does that end up working for you?