XF 1.5 Facebook Imported Image

Please accept my apologies if this is the wrong board / node. Im new, but will get the hang of it.

When I copy and paste a thread link onto my facebook group. The image Facebook uses seems to Vary. Sometimes it uses an Avatar, sometimes it uses the Logo, and sometimes it seems to use an image from within a post ??

Is there a set pattern to what is being used and is there anyway to get more control over which Image Facebook Uses. Ideally I would like it to be a picture from the first post or a Default forum picture ??