Click on an image to view the full size image. Right click on the image and select Save image as… from the dropdown menu.
Choose a location on your computer you can find later (desktop, downloads, etc) and click the save button in the lower right corner of the window.
Open MS Teams and start a meeting with yourself by clicking Meet now in the upper right of your window, then Start Meeting.
In the following video preview screen, click the Background Filters text to open the custom backgrounds menu.
Click Add new and navigate to where you saved the image and select to upload, all done! The background will appear mirrored/backward in your thumbnail, though others will see it correctly.