Does the fast break by the discharge of whitish-yellowish secretions—odorless—after suddenly seeing an indecent image and averting the gaze from it, knowing that nothing was discharged immediately after the image recurred in the mind?

The cure for waswas (whispers/obsessive thoughts) is to ignore them and not pay them any attention. Your fast is valid in any case. What comes out is not considered manī (seminal fluid) unless there is absolute certainty about it. The description of a woman's manī has been explained in previous fatwas. If you doubt what has come out, you may choose one ruling from what you have doubted.