Tonight I implemented something "just because" on my version of @Edent ActivityPub educational server. Some platforms will pick up a profile background image and display it along with your banner and avatar.
In your webfinger record, add this little bit at the root of the file and wallah. Akkoma is picking it up, at least! It's fun to play around.
```
"backgroundUrl": {
"type": "Image",
"url": "https://{your-domain}/{image}.jpg"
},
```