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 browserconfig.xml and 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)