Dear Mitchell82
Good day! Thank you for posting to Microsoft Community. We are happy to help you!
First of all, we apologize for the trouble and inconvenience caused to your work! Based on your description, I already have a general idea of what you mean, from the testing we've done so far, the path is: -%LOCALAPPDATA%\Packages\MSTeams_8wekyb3d8bbwe\LocalCache\Microsoft\MSTeams\Backgrounds\Uploads
The backgrounds won't automatically be ported across from the 'old Teams' location - %APPDATA%\Microsoft\Teams\Backgrounds\Uploads
Meanwhile if you still need further help, please kindly correct me and tell me more details so that I can do more research and tests, thanks.
Thanks in advance for your understanding! Your patience and cooperation will be highly appreciated. Hope you all the best!
Sincerely,
Kerry Chen | Microsoft Community Moderator