Setting a cover image for a list not only makes them look better, it's a helpful visual queue when viewing several lists.
To set a custom cover card for your List, first open your List.
Next, find which card you want to set as the cover and click on the card image. Click the three dots menu and then click Make Cover Card to set a card as your List cover photo.
From your List settings, you can also change between Art Focus and Full Card views for your cover photo.