It is present in post option, at least when you have an upgraded account.
Is it doing anything useful? Cause I've pressed it on post #259147 and it just disappeared.

If a post ends up with an incorrect thumbnail (occasionally posts have the thumbnail of an entirely different image), that fixes it. If the thumbnail is already correct, the link accomplishes nothing.

I've just refreshed page with ctrl+f5 and now I see that it indeed worked. (There was wrong thumbnail on this post when I pressed it)