Microsoft confirmed with an update to the original Office 365 notification (MC208577) that they will allow Teams users to upload their own custom background images in May 2020. However, if you can’t wait, you can upload your own images today.
On the PC you are using, go to; %AppData%\Microsoft\Teams\Backgrounds\Uploads folder and you should be able to select those images for a meeting background.
I have uploaded large high-fidelity JPEG photos to this folder and been able to use them as a background, but it seems like resizing the background to 1 MB or thereabouts is a good idea. Apart from deleting the image file from the folder, there’s no way to remove it from a PC after it’s uploaded. If you use Teams on several devices, you’ll also find that images don’t roam across devices. You must upload and maintain images on each device.
On Macs, the images should be copied to: