This is not about memory, it is about what you can access consciously. Those of us who have no image in our minds have memories of our houses, cars, friends, etc. just like you do (as far as I know!), they are just not part of our conscious experience. If you want to relate to this, think of language processing: you are not aware of how your brain parses the sentences you hear, though you still have access to the meaning that is produced by that parsing. We don't have access to the visual images, but they are there somewhere and can be used by our memory system. (ps. I suspect that getting images to reach consciousness or not is not just a random 'accident', but rather reflects a more generally different brain organisation -- just not wrt memory having access to visual representations or not)