Preview image for Safari search

When typing a website into the Safari address bar, you get potential matches popping up with an icon.


https://www.notestream.com/temp/Safari-Search-Icon.png

(Screenshot from iOS 9.1 simulator, iPhone 6s)


Experimentation suggests it is using og:image meta tag. Problem is, Facebook prefers a letterbox style image (much wider than tall). Clearly a square image is better here.


Is there a metatag that Safari will use in preference to og:image?