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.