Our modern representations of imagination usually involve things being emitted from or contained within our heads. (Thought bubbles, for example, and the many pieces of "imagination" clip-art where things are exploding out the back of people's minds.) This fits our modern understanding of our internal worlds as a product of our brains, which are located in our heads. I might hazard a guess that, since our eyes are also located on our heads, historical pictures of imagination might involve an analogy to vision and end up in the same place.
So, has the head always been the seat of the mind's eye? Are/were there any cultures that put it elsewhere? This is a very broad question that could potentially involve any culture's mythology, so I guess any concrete example of a story where the mind's eye was placed elsewhere (or the statement that there are none, if that's the case) would constitute an answer.