You can change page templates, but not image templates. This is frustrating.
You could, you just have to make the template selectable via a field in your file blueprints.
Possible but would mean adding the selector to every single file blueprint.
I’ve tried in many ways over the past, never got it to work. I’d love to see that piece of code, or better : native kirby support.
Related issue: https://github.com/getkirby/kirby/issues/1915