How does Facebook determine which images to use for thumbnails when posting links?

This features is arbitrary because the developers went for the “smart but not perfect” design style. An algorithm to find the perfect images to display as a thumbnail would be near impossible, so Facebook implemented the feature in this way. I assume even a list of sites with different algorithms is used, such as a YouTube video page being searched for only specific thumbnails related to the video. On other sites, images between a certain range of sizes are grabbed to give the user a small selection of possibly image thumbnails.

I just psoted an article from the NY Times today on FB and I was given a choice of thumbnails by the Times to use.