Change the background circle of a sun and clouds shown when at zero messages, to a photo of your choice.
Due to security restrictions on Chrome extensions, photos must be hosted at either Dropbox or Imgur, and due to security restrictions on Inbox, they must be HTTPS links.
Click the Google Inbox Photo Backgrounds button in the toolbar and copy-paste a link per line. When saving, it removes any invalid links.
To get a Dropbox link, generate a shareable link to the image, then change the bit at the end from ?dl=0 to ?raw=1 - i.e. https://www.dropbox.com/s/abcdefabcdefab/photo.jpg?raw=1
To get an Imgur image, right-click, 'Copy link address', then change the bit at the start from http:// to https:// - i.e. https://i.imgur.com/abcdefg.jpg