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. Open the Settings menu at the top of your main course page.
3. Scroll down to the file picker box labeled Course image.   
4. Drag and drop your banner image into the file picker box.         

Here's how my dashboard looks some days: