Idea: Zola could have builtin icon generator, that would create the various icon sizes and formats from a provided original file.
See e.g. https://realfavicongenerator.net/ for inspiration - in addition to the various file sizes, it also provides metadata files like
site.webmanifest that help various browsers and platforms display the site’s icon in the best possible way.
(note: not affiliated with that website/service, just an example that shows how much complexity there is that should be abstracted away from the end user)