Your XenForo homepage, threads, pages etc. have the ability to become Open Graphobjects. This can have numerous benefits, including the ability to specify an image to accompany any Like/Recommend news feed items posted to Facebook.
In order to participate in the Open Graph you must provide some extra information, which can take the form either of an Application ID, or the user IDs of one or more Facebook users who are authorized to act as administrators for the pages within the Open Graph. Both of these values can be set within the Facebook section of options within the XenForo Admin Control Panel.