The tv-config-toolbar-icons variable controls whether toolbar buttons display icons.
It can be set to yes or no prior to transcluding such a button.
The standard page template sets it to the value found in $:/config/Toolbar/Icons, with the result that this becomes the default for the whole page. The user can adjust this default by using a tickbox on the Settings tab of the Control Panel.
See also tv-config-toolbar-text.