Measure the width of the wall the rectangular picture frame is going to be mounted onto. Record the measurement in inches.
Divide the measurement you took in half. Measure and mark this spot on the lower edge of the wall or baseboard.
Dangle a plumb bob against the wall and line up the weight at the bottom with the mark you've made on the wall. Place the string against the wall when they're lined up and mark the wall along the string at the height where you want to mount the picture.
Place your picture frame mounting hardware on the wall at the point you've marked. The frame will be centered when you hang it on the hardware.