Apple - How can I set the background image the same on all master slides without editing each master slide individually in Keynote?

It appears the correct answer is I cannot do this unfortunately.

Create a slide with the background you want, then copy that background. Go the master slides section, click on the master slide you are using, and paste the background onto the master slide. All your slides (that use that master slide) will have the same background.

No, unfortunately there's no way to do such a thing (at least to my knowledge).

However, a quite effective workaround is to select all the items you want to apply to other master slides, by clicking all the items with or down, then copy them (⌘C) and paste to all other master slides. Luckily, Keynote doesn't move pasted items when pasting on another slide (as opposed to what it does when pasting the same item on the same slide).

Repetitive movement FTW:

  1. Click on the master slide that you just edited.
  2. Hit .
  3. Hit ⌘V.
  4. Loop.

Kind of ridiculous, but you can actually do it pretty fast ;)

(yes, it should be possible to automate this, but I don't feel like it's a common enough use-case to spend time writing an AppleScript)