[1.2.X] Fixed #11158 - get_image_dimensions very slow/incorrect after 1 call

  
Thanks to kua for the report, and to kua, SmileyChris and SAn for the patch

Backport of [13715] from trunk


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13716 bcc190cf-cafb-0310-a4f2-bffc1f526a37