The below screenshots are from the Windows version of Teams and Android App menu layout may differ on the Mac/iOS versions of Teams.
Changing your Team Picture on PC
Click the more options icon (highlight below) and select Mange Team
Move your cursor over the current Picture for the Team and click on the Pencil icon to Change team picture
You will then be prompted to Upload picture or Delete - Click on Upload Picture
A new Window will appear, navigate to the location where your new picture is stored, click on the picture, then click Open
Your new picture appears in the Change picture Window, click Save, you should see the image update then you can click Close
Changing your Team Picture in the Teams App on Android
Open Teams on your Mobile device and Navigate to the Teams Tab
Click the 3 dots next to the Name of the Team you are wanting to update.
The following menu should show, click on Edit Team
On the next menu, select Change team picture
This will give you the option to choose an image from your photo librarym, Take photo or delete the current picture
For this example we are going to select an image from the OneDrive already connected on the phone - select Open photo library
Navigate to the spot in your phone or connected OneDrive account
Click the 3 lines in the top left to show locations, then select your OneDrive
Select the picture you want
This will take you back to the Edit menu click the tick in the top right