I'm in the back, to the left.

is there a difference between #2 "to the left" and #3 "on the left"?

In this case of describing where someone/something is located, both are fine. (also used is toward the left). All are shortened versions of ...the left hand side.

I agree with missmiddlemarch, but would add that is could depend on how many people are in the picture. If there are a lot, "to the left" would mean closer to the middle to me than "on the left".

"on the left" sometimes means "on the very left". Hmmm ... hard to explain.

Essentially no big difference though, as MMM says.

Interesting. Thanks to both of you.