Reduce image size.

Checking the site with the PageSpeed extension, one of the reports was
in reducing image size.  This reduced the number of bytes by 67%.

It was done with optipng -o7 -fix *.png in the app/assets/images dir.

References:
  optipng: http://optipng.sourceforge.net/
  PageSpeed extension: https://developers.google.com/speed/docs/insights/using_chrome