Moodle randomly assigns images to our courses in the dashboard view like these: 

To replace a generic image with a custom banner: 

1. First, create a banner that is three or four times as wide as it is tall. (An image that is 1200 x 300 pixels generally displays well, but you may need to fiddle with the proportions to have the banner display correctly as it's not an exact science. Contact the CATL staff for assistance if you need help resizing a banner.)
2. Click on the admin gear in the upper right corner of your main course page and select Edit settings.
3. Scroll down to the file picker box labeled Course image.   
4. Drag and drop your banner image into the file picker box.         

Open your dashboard to see how it looks. To delete the current image and try again (if the image doesn't display as you'd like), click on the image in the Course summary files area, then click on Delete. Upload a new version and check it out on your dashboard again.

This is how this image turned out:

Here's how my dashboard looks some days:

Who has more fun than I do?