Deckard

Hello, we are offering a free online SaaS which also has the ability to upload pictures. This has now been abused for the first time by a 'spamvertisement' - so someone sending emails from a different server but in these email using images uploaded on our SaaS server. Is there a tool to reliably detect spamvertised images maybe on upload? Could this be solved server-side? Like blocking all images if they are trying to be loaded from outside our webpage? Any reasonable ideas would be appreciated. Thank you in advance Christian