Custom background policy is enabled? https://learn.microsoft.com/en-us/MicrosoftTeams/custom-meeting-backgrounds#enabling-the-custom-background-policy
Your custom background images are supported? https://learn.microsoft.com/en-us/MicrosoftTeams/custom-meeting-backgrounds#adding-custom-background-images
Can you use the official backgrounds from gallery? https://adoption.microsoft.com/en-us/microsoft-teams/custom-backgrounds-gallery/
Did you try the official "change teams background" methods and none worked?
https://support.microsoft.com/en-us/office/change-your-background-in-microsoft-teams-meetings-f77a2381-443a-499d-825e-509a140f4780 https://learn.microsoft.com/en-us/MicrosoftTeams/custom-meeting-backgrounds https://support.microsoft.com/en-us/office/customize-your-background-during-a-microsoft-teams-free-meeting-715a3e57-a0f1-44ee-ba9f-c9030888f0d3Please do tell what you already tried and what issues exactly do you observe.
Regards