On Google Classroom, you can change the background displayed at the top of the class stream.

As an administrator, go to classroom dot google dot 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 a 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.