From Lecture #3 in my video course for Based Deleuze:
What's really at stake here, I think, is the attack on representational thought... That's one of the core components of the Deleuzian project. Deleuze argued that any philosophy presents an image of thought and that this image of thought, it's not really explicit. It's never really demonstrated or proven. It's sort of a presupposition. Whenever a philosopher or any type of thinker or theologian or whatever presents a philosophy, there is in the background a certain image of what thought is and what thought should be, and what thought can be, and that's never really fully spelled out. It's never really justified.
It's essentially a kind of aesthetic. And there are different images of thought. This is something that Deleuze really wants to show to us… That we have a choice: an essential, irreducible kind of freedom or aesthetic decision to make about what type of thought we want to engage in.
In retrospect, "choice" is not the best word, because Deleuze wants to steer us away from any naive conception of free will. One is almost tempted to use an ugly deconstructionist term here, such as undecidability. The key point is that an 'image of thought' is extra-rational. It's never justified or formalized rationally, although it's implied in modes of justification or formalization. We might not "choose" our image of thought, exactly, although there is a kind of pre-rational selection process that sorts creators and their creations. Perhaps we could say that our 'image of thought' chooses us...