Optocoupler high side vs low side switch
Considering that particular type of optocoupler is effectively just a light controlled switch with no need of it's own supply or ground, it's conceptually like a relay, so you can place it anywhere in the circuit and it should still work. 

However, if the opto has any driver electronics (latch, logic gate, fet driver etc.), something that requires it's own supply, then things would be different. But a phototransistor by itself (which it looks like here) can go anywhere, the light provides the base drive.