Firstly, welcome in!
You will find the actual image inside the 'default shader' of the object. To find this, right click the object node and select 'internal network'. For objects, Terragen keeps the surface shaders inside the main node.
Another way to find it, if you're not keen on using the node network, is to open the 'settings' panel for the object and beside the name-field of the object you'll see a button with a miniature node network symbol on it. This will take you inside, as well.
Hope that is some help.