> So you can't accept a particular third party's images without also accepting its scripts, cookies, etc.

It's a clunky interface with poor discoverability, but with the uBO logger open in a browser tab you can click on any request right beside the timestamp and define a new rule with the desired granularity. In the URL Rule tab, the unmarked left column sets the allow/noop/block behavior.

Edit: Found the wiki page: https://github.com/gorhill/uBlock/wiki/The-logger#creating-f...