Yeah, it doesn't seem that merely blocking third party cookies can address a scenario like this, where the main site colludes with the third party tracking site

Yeah, it doesn't seem that merely blocking third party cookies can address a scenario like this, where the main site colludes with the third party tracking site. To avoid this, it still seems like the best approach is to use an ad blocker add-on like uBlock Origin, which will block any content from known tracking domains from being loaded. That should get rid of the third party image. There's also uMatrix (from the maker of uBlock Origin), which can selectively block images, scripts, etc. from third party sites.