Image protection on sub domain

We have a website on a windows server which uses images stored on another windows servers. These are accessed via a sub domain. We do not want to allow images to be accessed directly via a URL, they should only be viewable via the main domain name. Here is an example [login to view URL] the images on this page is held on another server and is referenced as follows < image source="[login to view URL]" / > (I have changed the code above so it displays , it should obviously say "img src") if the image URL is typed directly into a browser then the images shows. We want to prevent that happening so the image only shows on the actual website and cannot be viewed by typing the URL in We are using ASP and dedicated windows servers. We may consider changing the Windows image server for a linux one