A pre-created file might be fine. Personally I would advocate to generate them only at build time from the source PNGs, so that icns file would automatically update to new icon (sizes).

ECM has the macro https://api.kde.org/ecm/module/ECMAddAppIcon.html?highlight=ecm_add_app_icon for that, which also cares for Windows. Given Marble wants to stay ECM independent, perhaps good to look into copying that macro then instead.

Given this is a possible (no idea, cannot check :) ) improvement over the existing Marble.icns file in the repo, anything blocking this commit?
My previous comment was a proposal in general on the topic, but should not block intermediate improvements :)

This icon file still has rough edges. We have several other .icns icons in

src/mac/Contents/Resources/

in which the Marble icon looks pixel perfect. We need the same pixel perfection in this icon file as well.