It's like a photo. For me at least, it's kind of ghostly and transparent and appears briefly overlaid on what I can see with my physical eyes.
When I was younger the images were more solid and stable. Teachers often complained that I was spaced out because I was paying attention to some inner images.
That faculty greatly deteriorated and I became hyperverbal around the same time. (Coinciding, curiously with my shift from introversion to extroversion. Spurred, presumably, by my then-new interest in women!)
These days I can still access that previous level of visual fluency if I am in a state of very deep relaxation. (So it's conceivable that cortisol inhibits the circuits required to have such experiences. But I have no idea!)