Did you look at the Advertise With Us page (you access it by scrolling to the very bottom of any page in GR and clicking 'Advertise'. They have self serve ads which are the little 'sponsored book' ads and then is sounds like they have something they call their premium advertising which is probably the splashier ads that get prime real estate.
Are Goodreads Ads the pictures of new books coming out that I see on the side of the site and at the top as banners sometimes? I'm assuming yes but I'm not actually sure. I'm sure Goodreads would like a hefty sum or two for an author to do something like that.
Is that reserved for big publishing houses?