LeechBlocker is an open source ISAPI Filter for IIS.
The filter intercepts all incoming requests and checks to see if the resource (an image for example) is being referred to by the same site by which it is being hosted. If the referrer is not the same as the site name, the filter checks to see if the extension of the resource requested is an image (.gif, .jpg, .jpeg, etc…). If the extension indicates that the resource is an image, then the request is denied.
Read more at source: ISAPI Filter – LeechBlocker.