IIS 7 integrated mode & Authentication for all files handled by asp.net, but images are not being shown

I am trying to password-protect a site in IIS 7 with integrated mode.  The goal is to have all content password-protected, much like how you can create an IIS user and prompt for credentials before they can visit the site.  The only difference is that I am unable to create an IIS user, so I want to use integrated mode and authentication with asp.net.
What I've done that works:
First, I added forms authentication, which works for asp.net-handled items like pages with the .aspx extension.  (there is only one hard-coded user and password, so this should not be an issue.)
I added the following under <system.webServer><httpModules> section:
<remove name="FormsAuthenticationModule" /> <add name="FormsAuthenticationModule" type="System.Web.Security.FormsAuthenticationModule" preCondition="" /> <r ...