The custom maps can use ESRI REST Map Servers on the Internet or Intranet.

The general form of the URL needs to be : http://ArcGIS/rest/services/[SERVICE]/MapServer

You need to replace [SERVICE] with the actual map service requested.

For example, the ESRI map server for the ESRI World Shaded relief map is :

You can copy and paste this URL into the Add Custom Map dialog in GPS-Photo Link

List of Example Map Servers

Name Map URL
US National Transportation Map
US National Topo Map
US National Map Government Units
NY Ortho Photos 2010
ESRI World Shaded Relief Map