Test that an invalid source image returns a 404.
function testImageStyleUrlForMissingSourceImage() {
$non_existent_uri = 'public://foo.png';
$generated_url = image_style_url($this->style_name, $non_existent_uri);
$this
->drupalGet($generated_url);
$this
->assertResponse(404, 'Accessing an image style URL with a source image that does not exist provides a 404 error response.');
}