I'm sure either is fine, but I'm trying to understand grav best practices.
On one hand, themes makes the most sense because you are almost always going to have a theme head.html.twig template which will always reference the same icon files. But then, if you wanted to make changes, you'd have to go through the trouble over creating an overriding theme (mytheme) in order to override the theme's images directory.
One the other hand, user/pages is where a user should put his content. Since these icons will be different for each site...it seems they could also go into there.
Any thoughts?