On MathStackexchange, I guess they put ads up and a lot of them are for other websites, like the integer sequences website. And, the community gets to pick what the ads are. So, perhaps someone could start by nominating Sage. You would need to supply pictures for the ad by specifying a URL. Otherwise, I would just do it myself, but I'm not sure what to use.
http://meta.math.stackexchange.com/questions/6774/community-promotion-ads-2013
