I have encountered these expressions today, when I was describing a photo. People are lining up in the picture. I wanted to explain someone who is standing next to the person on the far left. And I started to wonder if there is any difference between 'the person second on the left' and 'the person second to the left'.
Let's keep this simple. When looking at a picture, the idiomatic expression is:
second from the left. = Starting from the left count to the right.
By itself, second to the left does not establish a definitive location. A particular point of reference must be specifically identified or clearly understood.
Second to my left identifies me as the point of reference. Count to the left of me.
A similar problem arises with second on the left. The expression on the left generally implies a particular frame has been divided into two parts: left and right. By itself second on the left creates a mental conflict, since second does not complement on the left.
Second on my left identifies me as the dividing line between right and left, allowing the listener to infer accurately the person second to my left.
If the people in this photo (source: Usual Suspects film) are numbered 1 to 5 from left to right, I would describe number 2 as second on the left or second from the left. That is, he is second from the left hand side of the photo.
However, if you asked "Which one is dressed all in black?" you could reply "The man second to the left of number 4." In this case to indicates direction, that you should go leftwards from number 4.