The only thing that comes to mind are spoonsorship deals with companies directly, so you put up a static image promoting their product for a certain amount of time, and they pay you for that in return.
Just like how offline ads work.
But it's untrackable, so advertisers (or PR departments) won't do that.
"But offline ads are untrackable!" Consider the sheer amount of CCTV cameras that exist just about everywhere nowadays.