gtk-drag-set-icon-widget (context widget x y)

context -- the gdk-drag-context object for a drag, this must be called with a drag context for the source side of a drag
widget -- a gtk-widget toplevel window to use as an icon
x -- an integer with the x offset within widget of the hotspot
y -- an integer with the y offset within widget of the hotspot

Changes the icon for a widget to a given widget. GTK will not destroy the icon, so if you do not want it to persist, you should connect to the "drag-end" signal and destroy it yourself.