But Caro isn't reproducing the image, merely hosting it, as part of a larger image-sharing site. The site has ads, which is how the image is monetized. Ben uploaded it without getting permission from anyone.
So Caro is getting money because Ben is using a copywritten photo for private use.
But Caro isn't reproducing the image, merely hosting it
A private backup service would be mere hosting. If Caro sends the image to third-parties, it's reproduction. Now, in the US, the DMCA law grants Caro a safe harbour, which is to say, a pardon for the infringement if Caro takes it down rapidly after being notified. But it's still an infringement.
Ben is also infringing, of course, assuming he knew the image was to be shared by Caro.
Well, I guess you could say that Ben is requesting Caro send the image to third parties - Ben's friends Darius, Esmre, and Fae - by linking them to the page on Caro's site that holds Aki's copywritten photo.
Also we'll assume that Ben waived rights to the photo on upload (as is common) and that he's one of those "No copyright intended" fools that also seem relatively common.
So Caro is getting money because Ben is using a copywritten photo for private use.