title: setting image resolution in Python pillow
body: <p>I am using Python Pillow to modify images.  Whenever I save a jpeg the internal resolution is set to 72dpi.  I am looking to see how I can set it to a different value.  I realize this is just a number and that in many ways its meaningless.  My motivation is to make follow-on work easier when I read the image into photoshop.  </p>