How do I disable Mojave's Finder quick actions for images?

keyboard shortcuts are usually managed in System Preferences > Keyboard, under the Shortcuts tab.

You don't say what the shortcut is, but a quick test reveals that it's Command R. (The Edit menu flashes, but there's no corresponding menu item.) Command Alt R goes the other way.

You may be able to assign that key press in the Finder to some other (rarely used) menu item or service, and see if that stops it.

As benwiggy says above:

You may be able to assign that key press in the Finder to some other (rarely used) menu item or service, and see if that stops it.

It does indeed. Before Mojave, Command-R activated "Reveal Original" in the File Menu. I created a shortcut in the Keyboard preference panel restoring that connection, and another one for Cmd-Opt-R to "Rename" in the File menu, which seems innocuous.