I've previously been using Default Shaders for moons and orbit shots, but I found that Image Map Shaders are simpler and give me better results for using surface textures. The issue is that I'm not seeing options for more than one type of map. At minimum, I'd like to add in a specular map to make the oceans on the Image Map texture look a bit more real. Ideally, I'd also like to use a Bump Map to give the map on a near orbit shot look a bit less flat.
Can this be done?
You can plug Image Map Shaders into the various "function" inputs of the Default Shader to use them as specular maps, displacement maps etc. If you just want colour and displacement, you can do that directly in the Image Map Shader, as it has options for colour and displacement. The Image Map Shader isn't designed to do every kind of surface attribute because that's what the Default Shader and Surface Layer are for.
In TG you can't apply bump without also displacing the surface, unless the object is an imported object rendered in ray trace mode. In that case displacement is turned into bump because displacement doesn't work under those conditions.
Matt