If you’re on a Pro, Conference or Teams plan you can utilize Unsplash for browsing and uploading beautiful pictures when creating your themes.
To add an image, select "Theme" in the top menu and then choose the tab "Unsplash", after selecting the option to add a background picture
1. Select "Add image" under Background Image
2. Select the tab "Unsplash" and browse pictures by typing in a keyword
3. Select the image you want to use and it will be added as a background image to your theme