Pay for journal cover image?
It's not that uncommon. Authors pay this charge mostly because they think having a cover image increases their reputation and/or impact. I don't know any study proving this assumption. Please let me know, if there are any.

Nevertheless, if an author wants to have a cover image, I always suggest to negotiate because this significantly decreases the amount of money the author has to pay, see e.g. 


  Similar story: Journal (a respected one) offered to use one of our images for their cover page. They wanted $1800. I said I don't have the money. They offered to do it for $900. I said no. In the end they put it on the cover for "free".


Source: Tweet be @rauscherMRI