To change this go to

onMessage > Content > Photos

  1. Select the Photo Category in question
  2. Select Settings
  3. Ensure Thumb is checked
  4. Adjust the size to desired size and Save
  5. Then re-uploaded the photo of the item that was not showing.

Please Note: Changing these settings will only affect new photos being uploaded to this category, any existing photos in this category will not be affected.