Function: turn-on-iimage-mode

Toggle Iimage mode on or off.
With a prefix argument ARG, enable Iimage mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil, and toggle it if ARG is `toggle'.

Uses keymap `iimage-mode-map', which is not currently defined.
(fn &optional ARG)