Measure your picture frame to determine the placement for your hanging mechanisms. Use a pencil to mark an "X" at three positions on the frame -- one at each corner, 1 inch in from the outer edge and 1/2 inch down from the top and a third at the bottom, centered across the width and 1/2 inch up from the bottom.
Locate the studs in your slanted ceiling with a stud finder, by running it along the ceiling. Measure and mark three spots on your ceiling to coordinate with the three spots on your picture frame; the ones on your ceiling need to be the same distances apart as the hanging spots on your frame. Position the spots on your ceiling on studs wherever possible.
Attach a keyhole picture hanger to each spot on the back of your picture frame. Position the hangers so that the larger portion of the keyholes face down.
Drill a small hole in each position you marked on the ceiling. Install a pan head screw into each hole.
Hang the picture on the ceiling. Line up the larger portion of the keyholes with each screw, then slide downward to lock the screws in the narrow ends of the keyholes.