Hello Albert,
Sorry to know that you are unable to fit the custom background in Teams. Let me help you!
I did my research and I found a thread that is similar to your concern. This customer found out that it has something to do with the camera aspect ratio of video as it was 4:3 and that is why the background was being cropped. It had nothing to do with the Teams application, rather with the camera their device. I suggest to resized the image and match the 4:3 ratio.
Source: https://techcommunity.microsoft.com/t5/microsof...
Let me know if this works for you.
Best,
Jayzar