Additionally I read about the following suggestions to display the images in gmail:
Including the scheme in the src url (using “//” does not work – use full scheme EG: “https://”)
Including width and height attributes
Including style=”display:block” attribute
Including both alt and title attributes
Old post but in case anyone finds this; we had the same issue with Outlook. Problem was we were using a Root Relative URL plugin which was removing the absolute image URL in the email. Deactivated that and problem resolved.