How can I get IIS7 to serve a default image instead of a 404 html page, when an image is requested?
request for: http://www.example.com/myImage.jpg
myImage.jpg does not exist, so we deliver another image instead?
Consider URL Rewrite or something like that, although you'll need to do some extending for it to know if the jpg exists or not.
You can use customErrors but that will affect all content type. Additionally, you could add your own HTTPHandler that watches for .jpg files, confirms that they don't exist, and serves up a default image it doesn't exist.