Oh no, you are not missing anything. When I found out you could make an image into a link I immediately started making games with pictures made out of several images pieced together, to make clickable areas. I have made some large games this way already, including one in which a piano has each key as a separate image playing a sound when clicked.
I assumed that the only other way to make point-and-click games was by using a game engine which I did not have, since I did not see anyone else doing it with Twine. Now that I know about image mapping I should be able to do a lot better! Especially since I just now searched image mapping and found this. My only concern is getting image mapping to fit in with the other things you can do with Twine, mainly the <<link>> macro. If that can work, then my life is changed!
Adding something to the game in a folder does not show up in testing, and since the audio and images and fonts I have made are what I would want to be testing I basically need to use urls for them all, at least I think so.
Thank you so much for your help!