Creating Avatar Borders or Profile Backgrounds

[Sadstronaut]

Does anyone know how new Avatar Borders or Profile Backgrounds are added? Are users able to create their own and upload them to the points shop?

[ReBoot]

Nope. But you can create a game, upload that and then upload whatever you want to the points shop.
You can also make your backgrounds available via market/trading/drops, the points shop isn't the only way to get new stuff.

[Mailer]

There is a bit of steamworks documentation that explains it to developers:
https://store.steampowered.com/news/group/4145017/view/2924488121712219461
However it is currently not for regular users to upload anything. You need to actually develop them with a game in mind.