Love the Siropu product and use it on 2 of my Xenforo forums. It lets them place the ad themselves (with your final approval), handles billing/inoicing them, reminds them when it is expiring/expired, serves it randomly (or not) in the positions you wish, keeps statistics for the advertiser to check at any time, coupons and qty discounts, optionally handles geographic or device restrictions for viewing, etc. They can set up the alt text, maintain their banner(s), geographic/device restrictions, the link it points to when clicked, etc.
We not only use it for advertisers, but for our own targeted marketing and decorations for every holiday. We have the potential advertisers assigned to a special VENDOR group, so only they have access to access the advertising system. (but you dont have to set it up that way). We target those people with special banners to get them to advertise, that regular users do not see.