Masons will tell you the keystone is not magic; it is simply the stone whose removal is unthinkable. Frames have one too, and the desk’s favourite judging game is to find it: cover the outfit, the setting, the pose in turn, and ask which single element the picture cannot lose.
When nothing is the keystone
If every element can be removed and the frame is merely smaller, the frame is a pile — competent, weightless, forgettable. That verdict is not a failure of taste; it is structural information. The frame needs one element strengthened until the others visibly lean on it: usually her face, sometimes the light, occasionally a gesture.
When everything is
The opposite failure is a frame where removing anything collapses it — overbuilt, every stone load-bearing, no air. Arches stand by arrangement, not by density. A frame that cannot spare a single detail is asking its viewer to do the arranging.
Build so that one element is the keystone and the rest are voussoirs: necessary, shapely, and honestly replaceable. That is the test, and it takes ten seconds per frame.