If you had pics of the whole heads rather than just the middle areas, it would look nice if you, perhaps, put them offset so that they are overlapping (which I assume that you are trying to do here), but as of now, it would probably be better if they were either:
A. in boxes
or
B. blended and blurred around the edges so that the cut-offs are not noticeable as a "mistake"
In the case that you want to do the overlapping, you will want to constrain your pics into the right 2/3 (or less) of the image, that way, it will look more like you are overlapping, and less like you are lining them up (as it does now).