do you want to put the image on every feedback popup or just add an image to a specific feedback message.

either way you will need to create a HTML image tag eg <img src="http://img4.wikia.nocookie.net/__cb20101002110826/callofduty/images/e/e8/Vault-Boy-yay.jpg" alt="thumbs up" title="thumbs up" />

If you want the image to appear on all tutor popups, then you will need to add the html for the image to the tutor template which is located at src/extensions/adapt-contrib-tutor/templates/tutor.hbs

otherwise you will need to add the html for the image to the feedback message