To change or add a profile picture to your 365 account follow the guide below
1. Open Teams app,
2. Click on your Profile initials to bring up the window then click on your profile initials on the new window
3. A new window will open click upload
4. Browse to the folder where your photo is located on your computer, select the image and click Open;
5. Select Save.
The image will synchronise to all your 365 apps