Changing the thumbnail for a lesson is possible using the following steps:
Login as an administrator
Navigate to the Manage tab
Navigate to the Lessons view
Select the name of the lesson you would like to modify the thumbnail for.
Select Edit Draft
Once editing the lesson, select the pencil icon, as shown:
This will display a modal window with the option to edit your lesson thumbnail.
You can now drag and drop your new thumbnail or click browse to select one.
Once your thumbnail uploads, select Save.
Next, Publish the lesson and you will see your new thumbnail in the lesson list.