Why must the image interact with the locus rather than just sit near it?

Interaction binds the two together; an image merely placed beside a locus drifts away and is lost on recall.

If a rake is simply "on" the doormat, the link is weak — you may reach the doormat and find nothing. But if the rake is strangling the doormat, or the doormat is choking on the rake, the two are fused into one event. Recall the place, and the action drags the image back with it. The rule of thumb: make them do something to each other, ideally violent, funny, or physical.