How can I change the default image appearing when no article thumbnail is set?

1. You need to login to the Magnet Publishers' Dashboard first.
2. Then navigate to the "Widgets" section. 
3. Click on the Global Variables button at the very bottom of the page.
   

     4. Enter the URL of the default thumbnail you would like to use accordingly.

Make sure the URL you entered is an absolute one. The image must load in the preview area on this form.