Configuration :: StyleSheetTheme Not Being Applied When Use The Local IIS Web Server?

My background image is not being applied when I switch over to using the local IIS server.

It's defined as:

<%@ Page Language="C#" StylesheetTheme="Common"...%>

VS Web Server: http://localhost:33174/Index.aspx = OK

Local IIS Server: http://localhost/MyApp/Index.aspx = background not showing

My Common.css class is defined as:

#masthead {
height:60px;
background-image:url(/images/bg_masthead.gif);
background-repeat:repeat-x;
}