Background Image Postbit

[Wolf]

Right I know some forums who have this feature but it's a custom plugin apparently and worth $80 lol I'm not bothered paying that much for just a plugin that changes background or you can add an image basically.

I know it can be done without a plugin but I just can't figure it out how, basically I'd like to have like specific groups that can add their own postbit background image near user details, the way for them to add an image is by going user cp > edit profile > and add an image link.. here is what I'm exactly trying to do..
https://community.mybb.com/thread-211061.html

Any help would be appreciated.

Thank you!