You will add them back by:
  1. Navigating to the Styles section of onMessage.
  2. Then Edit into any style.
  3. Click on any part of the style such as the container
  4. Click the Background Images Drop Down Bar on the right side where it says's --Please Select--
  5. Click the +Create New and then name and add the desired image
  6. Click Save
  7. Change back to none or leave as is.
  8. This puts an image back under Fonts & Images so you can add more from that screen now.