Migrating to Pantheon: How to make CiviMail editor insert images with non-relative refs?
When CiviCRM is on Pantheon, it needs a special set of config in the civicrm.settings.php which defines various variables to make CiviCRM work in cloud hosting. 

I had to add this variable to that list:

$civicrm_setting['URL Preferences']['imageUploadURL'] = 'http://www.mysite.com/sites/default/files/civicrm/persist/contribute/';


Then I had to make sure that the "system settings"->"Directories" was pointing to the same folder. Then it worked!