Your collection's cover image is what will show in search, on your profile page and if you share it on social media. To change it: 

• Open up your collection by clicking on it (all of your collections can be seen on the homepage). 
• Click the green 'Edit Collection' button in the top-right. 
• Click 'Add a Cover Image' or 'Edit Cover Image' if it already has one. 
• Click 'Upload an Image' to choose from your device, or 'Choose an Image' to choose from Unsplash, a free image library. 

On the right you can also choose to show the 'Full' cover image, 'Half' or none. This only affects what shows when inside your collection.