To enable a background image turn on your video camera and select the triple-dot icon. Select Show Background Effects
. From here you can blur your background or set the background to one of the available images.
You can also add custom images by places image files in a specific location on your computer.
Custom background directory on:
~/Library/Application Support/Microsoft/Teams/Backgrounds/Uploads
%AppData%\Microsoft\Teams\Backgrounds\Uploads
You may need to turn video off and back on to see newly added images.
This let's you do fun things like add a Max Headroom background.