I think you're sunk with what you're trying to do. The problem comes from the fact that the images are essentially in a different site. When you launch your site from Visual Studio, it creates a debug web server that runs just the site you're working with in an isolated environment.
One thing that I've done is to create a virtual directory that points to my web site. (e.g. I'll make a virtual directory in IIS called mySiteDev or whatever that points to C:\....\Projects\mySite\) Then, instead of running the debugger to test my code, I just point my browser to http://localhost/mySiteDev/. I usually stick with this method until I run into a situation where I need to step through code.
You might be able to get away with using ResolveUrl() in the code, but that could get messy and cumbersome if you have a lot of images.