I know you can specify specific images using open graph tags such as...

<meta property="og:url" content="http://www.yoursite/image.jpg"/>

...but does anyone know the logic facebook applies if you don't have this tag in place?  

Can't seem to fathom it out.

Seeing some sites where facebook doesn't bother showing any of the images, and others with a choice of up to 20.

I used to think it was the order in which the images appeared in the html, and never an image that was used as a background, although lately I'm not sure of anything about Facebook workings!. 

I do know that sometimes recently added images don't appear - I think FB uses a cached version of the page where it already has one.

thanks

I've got a case where none of our images are being brought in and can't figure out why.

It doesn't seem to be dimensions related.

Maybe its because they're served via a CDN on another domain?

don't think so, we have that on an ecommerce site and that seems to pull them in and cycle through everything. *normally* on that site it will select the largest image when a page is liked/shared

Although on some pages it is a pain in the bum and refuses to find anything but the least appropriate image on the page. I've now asked for it to be tagged up....