Hold the measuring tape at the top of the bottom-most stair. Measure up to eye level, about 60 inches and place a small, 1/4-inch pencil mark in that spot. The first row of pictures can be placed at a level below or above eye level, so long as the line is at the same angle of the staircase.
Repeat at the top of the staircase, measuring up from the top-most stair.
Hand one end of the measuring tape to your assistant and have him or her walk to the bottom of the staircase. Press the measuring tape against the wall so that both pencil marks are connected, creating a parallel line that mimics the angle of the stairs.
Place a 1/2-inch hash mark every 12 inches along the top of the measuring tape.
Line up the first row of your pictures so that the top two-thirds of each hangs directly above the hash-marked line, following the angle. Make sure that the lower-right hand corners of the frames are in line that is parallel to the hash-marked one. Adjust larger frames accordingly. Hang any additional pictures, whether larger, smaller or the same size as the others, above or below the first row in proportion to each other.