New York Timeslaid a big egg
[Y]ou have no legal obligation to contact the police, nor should you. The situation is too fraught with uncertainty. These photographs might depict--legally--not children but young-looking adults. The images could be digitally altered. Your boss may have acquired free (albeit illegal) images rather than bought them and provided a financial incentive to those who harm children. Someone other than your boss may have downloaded the pictures. ...

Since you have no reason to believe your boss has had improper contact with children, you should not subject him to such ferocious repercussions for looking at forbidden pictures.
I'm a janitor in a large company. When I was cleaning the president's office last week, I noticed the strangled body of a woman who appeared to be a hooker in the boss' closet. Should I call the police? J.M., Arkansas.

No. While strangling hookers is certainly wrong, there are too many questions here. How do you know the woman was a hooker? How do you know it was the boss who strangled her? Perhaps she hanged herself. If you call the police, your boss and your company could face extreme embarrassment and you could lose your job. Your best bet is to clean up the blood and stay out of it.
--Adam B. Kushner