How to add a default featured image to your Newsroom
Click on the user icon in the navigation bar...
...and go to 'Account settings':
There, choose the option 'Logo & Favicon'. You'll see the option to add a 'Default Featured Image':
Click the green 'Add' button and select an image from the media manager.
You can also click 'Upload' to select an image from your computer and then choose that image in the media manager.
Finally, click the 'Save' button at the bottom of the page.