Hmm......

|_____| = horizontal

___
|
|
___ = vertical......

So yea...I'm going with the "vertical" vote here. Good call, Cindy!

Nope, horizontal center. If the eyes were aligned in the center vertically, they would be the same distance from the top and bottom of the picture. But that's not the case. In general, most of the images shown are above the vertical centerline - but there isn't consistency among the images in this regard.

However, all the images shown have an eye that is centered between the left and right sides of the picture. That is, they are horizontally centered.

I can see your confusion, however. When marking a horizontal centerline, you use a vertical line! :^)