I suggest to implement easy background settings, that means, to upload background, and to decide how to stretch it according to CSS, like 2-3 options.
Will pay $10
Well, this is a "complicated" functionality because each theme implements background in its own way. GNU social's themes are mostly just css. We allow admins to edit the logo because that one is an HTML element. But background settings are directly "hard coded" in the theme's css, some don't even have a background image at all just a simple colour background.
In the following commit, and only for testing purposes, I added this directly in the core: 9a087ac6c5
Applying the same ideas, a plugin was easily achieved: https://notabug.org/diogo/gnu-social/src/background-plugin/
Solved with https://notabug.org/diogo/gnu-social/pulls/61 . The bounty can be sent to https://www.diogo.site/support