On Google Classroom, you can change the background
displayed at the top of the class stream
as an administrator.
Go to Classroom.google.com.
Click the class, and at the bottom of the image,
click "Select theme."
You can select an image from the gallery,
and then click "Select class theme."
You're also able to upload your own image.
Click "Upload photo."
Drag a photo from your computer to the middle of the screen.
Or click "Select a photo from your computer,"
choose the image, and click "Open."
Finally, choose "Select class theme."