If you are experiencing comments or have seen images you deem to be inappropriate you should contact the social networking site directly using the self-reporting facility usually via the help or support centre on the web page.
Anything you deem to be threatening, harassing or of a criminal nature should be reported to the police and we would ask that any evidence be captured as soon as possible, you shoud consider the following:
CHAT LOGS/IMAGES/WEB ADDRESS - cut and paste or 'save as' into a word/text document - you could also try the print screen facility.
You could also try and take a photograph of the screen or leave the page on the screen for the police to view.
GENERAL ADVICE - Make sure to use strong passwords usually a combination of upper case, lower case, symbols and numbers is best. Apply privacy settings to Facebook and use reputable anti-virus/spam protection which should be run regularly.