Don't cling to anything and don't reject anything. Let come what comes, and accomodate 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. Don't fight with what you experience, just observe it all mindfully.
Bhante Henepola Gunaratana, Mindfulness in Plain English