Let come what comes, and accommodate yourself to that, whatever it is. If good mental images arise, that is fine. If bad mental images arise, that is fine, too. Look on all of it as equal, and make yourself comfortable with whatever happens.
Bhante Gunaratana, Henepola Gunaratana (2011). “Mindfulness in Plain English: 20th Anniversary Edition”, p.34, Simon and Schuster