This is from a blog post that Scott Guthrie wrote on URL Rewriting (original post here: http://weblogs.asp.net/scottgu/archive/2007/02/26/tip-trick-url-rewriting-with-asp-net.aspx)

Handling CSS and Image Reference Correctly

One gotcha that people sometime run into when using Url Rewriting for the very first time is that they find that their image and CSS stylesheet references sometimes seem to stop working.  This is because they have relative references to these files within their HTML pages - and when you start to re-write URLs within an application you need to be aware that the browser will often be requesting files in different logical hierarchy levels than what is really stored on the server.

For example, if our /products.aspx page above had a relative reference to "logo.jpg" in the .aspx page, but was requested via the /products/books.aspx url, then the browser will send a request for /products/logo.jpg instead of /logo.jpg when it renders the page.  To reference this file correctly, make sure you root qualify CSS and Image references ("/style.css" instead of "style.css").  For ASP.NET controls, you can also use the ~ syntax to reference files from the root of the application (for example: <asp:image imageurl="~/images/logo.jpg" runat="server"/>