Rule: Use OpenGraph Tags
Open Graph Tags are important for special services of bigger websites. The most known site is Facebook who used them to automaticly post meta information on your wall if you push the Like Button of a website.
The og:image attribute is not required by open graph itself but it is highly recommended to use it. You should also use it if you got no implementation of Facebook Like because all major fetching services will use them as preview image.
Example on missing tag: A website will be shared by some user in his facebook profile like posting a link instead of a status update. If there is a html code which definies other article images above the first article image, the preview image in facebooks timeline an show up an image from an other article instead the original linked article. This exampe was originaly seen on a big news publishing site with a linked political article and a shown erotic image as the preview picture.
Following tags are required:
og:title, og:type, og:image, og:url, og:site_name
For further information please look at the Open Graph protocol documentation on Facebook.