There are some sites that just use images, but in dollar terms the overwhelming majority of ads are served by ad networks, especially Google AdWords. Ad network

There are some sites that just use images, but in dollar terms the overwhelming majority of ads are served by ad networks, especially Google AdWords. Ad networks create a large and standardized ad market with low transaction costs. By using a network, the content site and the advertiser don't have to spend many hours to find each other, negotiate terms, and figure out how to trust each other. Further, ad networks provide quantitative analytics and targeting information based on sophisticated models of user behavior. These features represent huge investments of data and expertise, which individual sites and advertisers don't have. A local business can spend 1 hour and $500 on Google AdWords or Facebook Ads and be fairly confident that their ad was seen by X number of real people in their community. A larger brand can pay $500k to run very sophisticated campaigns, where they can drill into detailed demographics, A/B test their ads, and mock up ads to get real-time feedback on products they haven't even made yet. Even with rising adblocker rates, ad networks are still more efficient for most advertisers and content websites. (I'm not affiliated with Google or Facebook.)