Hi,
In IIS, .NET will not handle requests for images, css or javascript or pdf or most any kind of file except .NET specific files like .aspx, .ashx, .asmx, .config, etc. For all files not specifically handled by .NET, IIS will handle those requests (much more efficiently I would add) and the request never invokes the ASP.NET pipeline. Now things may be changing a bit in IIS 7 as they are adding features I think to make it easy to handle more files types in .NET if you need to, but I think the default would still be not to handle them. There are some cases where one might wish to be able to handle these requests in .NET. In IIS 5 or 6 it takes third party isapi filters to do it but I think they are making it more easy in IIS 7. Still, in most cases you are better off letting IIS handle requests for these kind of files for best performance.
Best,
Joe