THAT's your behaviour. We can all observe that. That is imagery, but only you can observe whether or not it is happening. That is introspection. This is what Kosslyn shows, as you will see in this week's reading about the imagery debate.
Brain activity is observable, like behaviour.
Don't confuse mental imagery with brain images. The essay question was about brain images. The mental imagery topic is next week. The behaviour is what everyone can observe. Only YOU can observe what you imagine.