Nevermind.....I answered my own question.
To allocate service providers to ArcGIS Server Image Services that are created from the iscdef files on your image server you simply edit the ImageServer.Services file in Notepad or an XML editor. The file can be found here:
C:\Documents and Settings\All Users\Application Data\ESRI\Image Server\Server
This file lists all services running on the Image Server and also provides a tag for <ServiceProviders> which you can populate with the names of the Service Providers you want to allocate to specific services. I edited this file and saved the changes when the Image Server was stopped, once restarted the changes were implemented and my ArcGIS Server Image Services were assigned to a specific Service Provider.