got the same question actualy... I have posted links of the forum in facebook page and it shows Xenforo logo.
I noticed other links from other site shows the image that relevant to the link itself. (see attachment- the first post is the link from my forum, the second one the link from other site)
I see Mike suggestion to edit the open_graph_meta template, where can I find that template?
I believe this is quite important feature as someone might interested to follow the link if see some images that catching the eyes.