views.py

class ImageQuerysetMixin(object):
    """
    Make sure to use the correct arguments for getting a single image.

    With this mixin it's not possible to request an image just by its
    primary key, we'll also need the correct username.

    """
    def get_queryset(self):
        return self.model.objects.filter(
            pk=self.kwargs.get('pk', None),
            user__username=self.kwargs.get('user', None)
        )