[1.2.X] Fixed #14430 - Test failure on Windows with get_image_dimensions since [13715]

Thanks to gabrielhurley for report and patch.

Backport of [14001] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14002 bcc190cf-cafb-0310-a4f2-bffc1f526a37