Hi, new here.
I am utterly incapable of forming voluntary mental images, and experience very faint involuntary ones only occasionally, during the hypnagogic state when falling asleep. (I used to practice at manipulating these, but made no headway.) I do experience afterimages, and I must be encoding information in a 'visual format' somewhere, because I can rotate molecular models (for example) in my mind with no problem, and get a very faint disturbance in my visual field when I do so.
Yet I do dream, sometimes quite vividly. Dreams are pretty much the only time I see something purely in my mind. I once experienced bizarre visual hallucinations due to a side-effect of medication, and they struck me as being quite dreamlike.
I suspect that my incapacity for mental imagery was strongly influenced by the fact I was born blind, and had no usable vision until the age of three. However, so far as I know, that doesn't explain my incapacity for other kinds of sensory imagination.
I am a fairly skilled singer, with a good pitch sense, yet I would not say I can 'hear a tune in my head'. Rather my experience is that I 'just know' what intervals sound like, how the tune flows. I can hum or sing it for you from memory, but I cannot 'play it back' in my mind. When I try, what I really end up doing is making motions in my mouth and throat as if I were singing very faintly. It's as if the information is encoded somewhere, but gets decoded only at the point of action. In much the same way, though I can't draw well, I can roughly draw complex shapes from memory - like the outline of the contiguous United States. But I am not aware of experiencing that shape in a visual way in my mind; it is somehow encoded.
I used to believe, as this excellent post says, that my experience was universal and that all talk of 'visual imagery' was metaphor, but I was convinced otherwise by deep conversation with a close friend who is an eidetic imager.