LeechBlocker is an open source ISAPI Filter for Microsoft`s Internet Information Services by Michael R. Brumm. This Isapi filter can be very easily added to any IIS web site, and can protect the whole server or just a single site from leeching.  

The Problem

One very annoying thing about hosting images on a web site is that certain people (leeches) will take advantage of the fact that your web server will serve the images to any browser that requests them, regardless of whether the browser is displaying a page on your web site, or a page elsewhere on the Internet.

Because of this, the leeches will create their web pages in a way that display images which are retrieved from your web server, not their own. By not hosting the images themselves, they save both disk space and avoid bandwidth limits on their web server. This strategy is sometimes called leeching, hijacking, or stealing images.

Unfortunately, this means that your own web server ends up responding to a lot of leech requests which have nothing to do with your own web site. This will chew up your server`s bandwidth and may slow down your server. In addition, it can skew your log file analysis of visitor usage and browsing.

How The LeechBlocker Filter Works

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.

More information & Download

http://www.michaelbrumm.com/leechblocker.html