gtk-tooltip-set-custom (tooltip custom-widget)

tooltip -- a gtk-tooltip object
custom-widget -- a gtk-widget, or nil to unset the old custom widget

Replaces the widget packed into the tooltip with custom-widget. custom-widget does not get destroyed when the tooltip goes away. By default a box with a gtk-image and gtk-label is embedded in the tooltip, which can be configured using the functions gtk-tooltip-set-markup and gtk-tooltip-set-icon.