A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
In my experience, PPT only returns to the last-used folder during the current session.
Quit, restart and it returns to whatever folder MS has decided we should all store our pictures in.
And even for the current session, if you accidentally get it set to some other folder, you'd only need to insert an other image and browse back to whatever folder you want; that'll reset it for the current session.
Why muck about with the registry.
If you want it to stay put from session to session, I have an inexpensive ($20) add-in that inserts pictures, remembers the last-used folder and file-type automatically and even sets the picture name = the original filename so it appears as Whatever.JPG instead of Picture 42 in the animation pane.