This is a picture hanging system specifically designed for the IKEA STROMBY range of picture frames. The minimum you need to print in order to hange a typical A4 sized frame is two "Picture hanger top Angled" pieces. These are very simple items, basically a stepped cylinder with an eccentric  (countersunk) hole through it. These two items are screwed to the wall, with the larger diameter stepped out from the wall. Because the hole is eccentric, any slight inaccuracy in level can be compensated by losening the screw and rotating the cylinder slightly so that the stepped out part becomes level with the other one, then tighten up the screw.

If this is to be used in an environment where people may try to remove the picture (say a pub, where this was designed for) you will need to print one (or two for large pictures) of the "Picture hanger bottom" piece. These stop the bottom of the frame from swinging out from the wall. To locate them, hang the picture, mark the bottom of the frame on the wall, remove the picture, and then screw the bottom piece onto the wall such that the lower part of the center curve touches the mark on the wall. It is essential that the SCREWS ARE NOT TIGHT. the plastic must be able to rotate round the screw when the center of the curved piece is lightly pressed upwards.
To hange the picture, engage the bottom of the frame with the bottom piece, press up slightly so that the bottom piece flexes just enough to allow the top edge of the frame to 'hook' onto the top pieces.
To ensure that the frame cannot be removed from the wall, you can screw the '"picture hanger lock" piece above the picture, finishing it off with the small plug in the screw hole.

The top piece has the angled step because when the wall is not totally flat, it is sometimes difficult to get it to fully engage behind the picture frame.

If the frame is holding a particularly thick display piece (say an image on thick cardboard) the bottom piece can be difficult to engage, I have other designs to to cope with this eventuality.

I printed these in PLA and are using them to hold up the largest STROMBY fames in a pub.