By the way, I just "fixed" the Thunar problem by passing the png's to `convert` in reverse order. I just repushed with this version.
Looks like compressed icon (in this case that 256*256 one) can not be last in .ico file, otherwise gdk-pixbuf can not properly read that file (because that libarary still don't support copressed icons). [.ICO files are displayed as noise](https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/issues/86)