How to put pictures into unusual shapes, using PowerPoint

This article explains how to put a picture that is one shape (a "square peg") inside an image of another shape (a "round hole" - or star, elipse, octagon, etc) - using Microsoft PowerPoint.

Original title:   How to put a square peg into a round hole - in pictures

Recently, I've been using PowerPoint to turn into the thumbnail image in support of my posts. This capital so as to I own the copyright of the pictures, so can share them lacking worrying almost copyright issues.