I haven't even known how to search for this, but you know when you post a link in facebook or some other social network, and it takes one of the images from the page and creates an icon of it to go with the post? Well I was wondering how the image is chosen, so that I might be able to control what the remote site sees. I would especially be interested in supplying a default image for when a page might not feature any images at all.

If anyone knows what this process is called, please let me know so I at least know what to google for

Thanks.