Changed `not foo == None` to `foo is not None` in image sniff handler

Changed `not foo == None` to `foo is not None` in image sniff handler

  >>> class foo(object):
       def __eq__(self, other):
           return True

  >>> f = foo()
  >>> f == None
  True
  >>> f is None
  False

-- <http://stackoverflow.com/questions/26595/is-there-any-difference-between-foo-is-none-and-foo-none>