Your background-image url must be an endpoint that can process a request. Simply, requesting a placehold.it/a image is pointless, but sending to l33thacker.com/a, assuming that l33thacker.com knows how to process that request maliciously, will work.
But you would still see the network call to placehold.it/ if it actually worked.
It can be done manually. Send the request to a server you control and check the logs. It doesn't scale well but it would work fine for a targeted attack.