Apparently, one can refer to a program's icon solely by its name, like here:
zenity --question --text "Hello <b>world</b>" --icon-name "baobab"
chk_icon "baobab" ; $?=0
chk_icon "installed stuff" ; $?=0
I was able to add a new recognized icon name by copying my icons to the various /usr/share/icons/gnome directories, by size, and THEN running sudo gtk-update-icon-cache /usr/share/icons/gnome without which the icon will still not be recognized.