That's the downside to using an outside photo host. Back in "the day" when you basically needed your own host for pictures, a friend of mine found his pictures being used heavily on other sites. And he was paying the bandwidth.
He put a routing rule in his web server that if the calling URL was not on his server, then the server served up some pictures of condom packages. Stopped the problem!
He put a routing rule in his web server that if the calling URL was not on his server, then the server served up some pictures of condom packages. Stopped the problem!