Profile banners let members add custom background images to their public profiles. You can turn this into a paid feature by taking it away from regular members and selling it back to them. Start by changing the Registered group settings. Go to Groups & permissions, click on User groups, select Registered, and change Upload a profile banner to No. Save these changes, and nobody can use profile banners anymore.
You need a special group for people who pay for banners. Create a new group called Has profile banners. Find Groups and permissions, click User groups, choose Add user group, and type the name. In the permission chart, Set Upload a profile banner to Yes. Save everything when you finish. This creates your special group with banner...