Symptoms

When commenting on a JIRA issue and including images (i.e. jpg, png, etc), the notification will be sent but the image(s) will be displayed in your email client as broken thumbnails. 

Cause

This is the intended behavior since you need to be authenticated to see the images. More specifically, the email client/program tries to load the content server side but the images physically reside in the directory structure of your JIRA Cloud virtual machine.