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 :

http://services.arcgisonline.com/ArcGIS/rest/services/World_Shaded_Relief/MapServer

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 http://services.nationalmap.gov/ArcGIS/rest/services/transportation/MapServer
US National Topo Map http://services.nationalmap.gov/ArcGIS/rest/services/US_Topo/MapServer
US National Map Government Units http://services.nationalmap.gov/ArcGIS/rest/services/govunits/MapServer
NY Ortho Photos 2010 http://www.orthos.dhses.ny.gov/ArcGIS/rest/services/2010/MapServer/
ESRI World Shaded Relief Map http://services.arcgisonline.com/ArcGIS/rest/services/World_Shaded_Relief/MapServer