Drupal image styles (formerly image cache) and image quality issues
Most people with image style quality issues look to the "JPEG Qaulity" setting (admin/config/media/image-toolkit in Drupal 7).
I have found it much more usefult to upgrade to the ImageMagick module. You can set ImageMagick as the default image style toolkit in the same admin path as above once the module is enabled.
Photo credit: http://buscarello.com