Your Collection's background image is what appears behind the content in the middle. 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 Background Image' or 'Edit Background 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.