The X-standard xwd utility nicely complements Tcl in at least a
couple of ways.  It can do useful [image manipulation without Tk],
and in particular it can be a handy way to
convert formats.  [DKF], for example, suggests
      # Does not work if anything is overlapping the canvas,
      #     and only does the currently visible area.
    exec xwd -id [winfo id $canvaswidget] | convert - foobar.png




[[But only available for X, that is, essentially, Unix.]]