def METHOD_NAME(self):
    img_src1 = ImageSource(Image.new('RGBA', (100, 100)))
    env = make_wsgi_env('', extra_environ={'mapproxy.decorate_img': self.return_new_imagesource_callback})
    self.new_img_src = None
    img_src2 = self.server.decorate_img(
        img_src1, 'wms.map', ['layer1'],
        env, self.query_extent
    )
    assert img_src2 == self.new_img_src