As soon as ads are shown on the page, it's typically considered commercial, at least in Germany (but probably throughout the EU).
Also, Facebook, Google et al usually have ToS that allow commercial re-use. So sharing your personal photos is possible - so posting images with any kind of copyrighted scenery to such services (likely includes Twitter and Instagram, too) becomes a legal minefield for the user, because they may be liable if the platform operator ever chooses to use such an image.
So does that mean that you're not allowed to use the images in places where other people would make money?
Most copyright laws assume there are just 2 players: Aki who owns the property and the copyright, and Ben who takes & monetizes the photo and then pays royalties.
In my scenario there is a 3rd: Aki owns the property and copyright still, and Ben still takes the photo and posts it. But then Caro monetizes the page that the photo is on.
Does Ben owe royalties to Aki? Does Caro? Do both? Neither?
Most copyright laws assume there are just 2 players: Aki who owns the property and the copyright, and Ben who takes & monetizes the photo and then pays royalties.
Usually it's the reproducer who has to pay, so Caro would have to pay both Ben and Aki, for the photo and property licenses respectively.
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.
Also, Facebook, Google et al usually have ToS that allow commercial re-use. So sharing your personal photos is possible - so posting images with any kind of copyrighted scenery to such services (likely includes Twitter and Instagram, too) becomes a legal minefield for the user, because they may be liable if the platform operator ever chooses to use such an image.