UI to override featured image display on single posts is not clear

[aschweigert]

In the modal where this is display it's so tiny and at the bottom of the window that it's easy to miss. We should consider adding it either at the top or maybe in the right column with the photo info so that the context is clear.

We might also want to add some explanatory text to make it clear that this means it will be hidden in the hero area of the single post but still used as the thumbnail on the homepage and elsewhere on the site.