Before anything, I have to say that I do not advocate uncritical use of this solution. A good system should validate all input and keep its entities valid at all times. If you end up needing this one, you shold be needing it because you're creating...
I've been sure the ASP.NET theme framework fixed the url's of pictures with equal paths under the application as the themes. Looks like it doesn't. The only way to theme images is to have a skin file in each theme. Why would I want to duplicate...