Microsoft 365: Change your Display Picture
Open the Teams app.
Click your initials or current photo.
Click Change picture.
Click Upload picture.
Locate the picture you want to use, and click Open.
Click Save.
NOTE: Your profile picture will display throughout Microsoft 365. It may take a few hours for your photo to display in all Microsoft 365 apps.
TIPS
Square pictures that clearly show your face work best.
It is recommended that you have a relatively plain background to allow people to see your image clearly.
There are no image size restrictions, but remember that your profile pic will display at about 95x95 pixels.
(Shamelessly stolen from Texas State University here)