Well, that's a different proposition: I think that if you were to query you'd find that people who experience strong mental imagery have done so as long as they can remember, and don't have the experience of needing to develop it as a skill. It's true that such data isn't in Galton's study; perhaps it's in others, but more simply, we can ask:

Anyone reading this who has strong mental imagery (video like): Do you remember ever not having it? consciously or unconsciously, did you ever have to work to develop it?