[LinuxCNC/linuxcnc PR#548] bug: fix import of ImageTk so that it works with python-pil

Someone has already updated the other Image imports so they work with both the python-imaging and python-pil packages (python-imaging is no longer available in some Ubuntu based distros), but this one was missed causing image-to-gcode to crash when loading a file.

This needs to be merged up to master.