Thumbnail on new tab page not showing up

Well this isn't a huge problem, in fact I don't mind it much, I'm just wondering why this is happening:

Upon opening a new tab page, thumbnails usually show for your 9 "top sites". For me however, they usually show up, but on a particular website, it is not. I have no idea whether it has to do with Firefox or the website, but I figured it was worth asking it on here. (the website is

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