Hello Kayla_660,
I am a Microsoft user like you, providing solutions to community members; I am NOT a Microsoft employee.
Unfortunately, there isn't a direct way to lock the Teams background to a specific aspect ratio (horizontal/vertical) and prevent it from automatically cropping to a square during calls. Microsoft Teams automatically adjusts the background image to fit the video aspect ratio, which can be frustrating when using a branded background with a specific design.
Although there's no straight forward resolution to this issue, I have found a Microsoft Teams thread where similar issues were raised: https://techcommunity.microsoft.com/t5/microsoft-teams/thread/m-p/2243728
There was a particularly helpful comment from (thepigshaveflown) you can try out "My IT guy managed to find the problem for me. It was the camera aspect ratio.
I went to the camera app of my laptop and in settings, I found that the aspect ratio of my video 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 of my laptop. So, I resized the image I wanted to match the 4:3 ratio and now it is perfect."
Hope this helps.
-Stephen N.