I suggest you just start describing. Rather than an after image ,pick something that you like and know fairly well. Perhaps a flower, your favourite car and then pretent you're describing it to someone. Think of the details as clearly as you can. If you're like me you have to be quick because the image seems more like an ideas rather than images that hang around for description. So decribe the ideas, forget about waiting for 3D pictures in full technicolor. Just describe what the mind seems to tell you to.
This method has give me some great insights from my inner mind.
Alex