Originally Posted by
krabbenkopf
You can upload your own backgrounds to %APPDATA%\Microsoft\Teams\Backgrounds\Uploads.
Microsoft will add functionality to add your own backgrounds from within the Teams client in the next couple of weeks, but as
krabbenkopf points out, you can DIY in the meantime, any images you add in the uploads folder will appear at the bottom of the list of M$ supplied images. This does not work on all clients, Linux, Windows 7, and some with older graphics cards.