I have a page on my site, for purposes of this example lets say http://myhost/Pages/Default.aspx. On Default.aspx I have a user control called MyControl.ascx which lives in /Controls/MyControl.ascx. So the tree looks something like this
You have a bunch of options:
You could hardcode the Url using the
~ operator which give you the root and then define it from there like:
~/Pages/AboutMe.aspx. Keep in mind that the
~ operator is recognized only for server controls and in server code.
You could also use the
.. to get you to where you want as it will navigate back up the folder structure like:
HttpRequest.ApplicationPath will get your the virtual application's root path on the server which you can use to build off.
For more information on Pathing options you should read this article on MSDN:
EDIT: Based on your edit to your question, you should then pass in the relative URL into you user control via a property. Let the page that uses the control define the relative path to the resource it will require.