This has been done before (and posted to HN), and it had the same problem. Way too many false positives, especially with older sites.
For this service, if it can find an image, it's at least an image the site owner wanted to be used to represent it.
<meta property="og:image" content="//nebula.wsimg.com/7cdaeb9fbb87f3450dc65108e6d6af87?AccessKeyId=6E4E9B5CC8D37F66F76C&disposition=0&alloworigin=1">