I am having the same trouble. I appreciate that Guy is distinguishing between Imagine and Mental Image for us to make it clear that they are different for him, but I'm not sure exactly what the distinction is. Especially for something so visual like, "Tom Cruise's baby face."

When I imagine that I automatically picture something. I think of images I've seen of Tom Cruise in movies or on movie posters. If I try to focus on just his face in isolation of any surroundings, then my image starts to feel more blurry. I think there's a limit to the resolution of my mental imagery -- and it seems that imagining the context as well helps -- but I don't think I can imagine things without picturing them at least to some degree.