then will clearml associate that image with my experiment and always use that image with it,

when you say "agent to use my docker image," I'm assuming you mean the configuration file or --docker argument, in both cases this means Default conatiner.
This means that if the Task does Not specify a docker, it will use the one you set in the conf/argument, But Tasks can always specify a different docker to use, and the agent will pull the requested docker based on the Task's entry.
Even when you clone a Task you have the previously used docker image set, but you can still change this value in the UI.