A banner image is a decorative picture at the top of the course page.
To add a banner image to your course, go to the course and turn on Edit mode
In the top section, click on the 3 vertical dots in the upper right corner of that section box, then select Edit Settings
In the Description box, click on the Insert/edit image button on the toolbar (looks like a picture with a mountain and the sun).
Click Browse repositories... , then click Upload a file to upload a file from your computer or use Server files to select an image you have already uploaded.
A preview of the image should now appear. Add an image description for users with screen readers. Adjust the size and alignment as needed Generally, you will want an image 600 pixels or less wide to avoid display problems.
Click Save image at the bottom of the popup window to add the image.
Then click Save Changes to return the main page of your course.