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