the easiest way is to turn snap on, drag a ruler vertically to the center, it will snap, do the same for horizontal. then select the layer with your image and using the move tool, move it so the circle with the dot in it is locked into the center of the horizontal and vertical ruler lines.
Hahaha, I've actually always wanted to know this too, I think it's kind of ridiculous that they dont have an actual feature for exactly this
