Este script python por cheshirekow afirma fazer o que você quer.
#! /usr/bin/python
import pygtk
pygtk.require('2.0')
import gtk
import os
import sys
def copy_image(f):
assert os.path.exists(f), "file does not exist"
image = gtk.gdk.pixbuf_new_from_file(f)
clipboard = gtk.clipboard_get()
clipboard.set_image(image)
clipboard.store()
copy_image(sys.argv[1]);