Created by Myles Johnston, this ScummVM fork lets you insert your own graphics (backgrounds & sprites) into old Sierra adventure games.

Check out this tutorial:
https://youtu.be/qoRhbdoaSqA

Spece Quest Historian made a video about this:
https://youtu.be/piFSUpRhfeU

Finally, I can have King Graham become Cedric the Owl and Cedric the Owl become a very tiny Graham imp.

It works only for backgrounds, not characters.

Edit: there's some conflicting info circling, so I'm not sure what you can actually use it for. Maybe you can even swap some character skins, but not change their size.