Right now, if there's a Flickr image that's missing, the file check actually pulls down Flickr's "image is missing" image, and thus it isn't reported as missing. It should be. Not sure there's a 100% foolproof way, outside of maybe tying in some image recognition, but a quick proxy would be "do the image dimensions match the dummy image and is the filesize right?" Note that I'm not sure the file size is always identical, but it's always < 5 KB for a decent size image.