Optimize PNG images

Run the PNG images through a loseless optimizer to reduce size.

Test: git difftool m/master -x 'compare -metric AE $2 $1 NULL: || :'
      The diff command returns the number of pixels that are
            different for each image, and are all 0s.

Change-Id: If48e0b4a1e1e34a2d9b31ee88ee07ac46536483c