What is the physical path to my web site?

You will need to navigate the Helm control panel as follows:

Domains -> Select the desired domain -> Web Site Settings -> your path will be displayed next to the physical path label.

The other option you have which is the preferred way to determine your physical path using ASP code can be found below.

<%
   Server.MapPath(".")
%>

or

<%
   Server.MapPath("/img/testimage.gif")
%>