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.
See this video for more: