It appears as if many Facebook users are seeing images in their news feed that are more than a little graphic. Users are reporting seeing images of explicit violence and pornography. Obviously, these are not images that fall under the standard Facebook terms of service, and therefore should not be showing up in profiles.
As of this point the reason some users are seeing these images is not perfectly clear. Experts are thinking that this may be a form of “clickjacking.” This is a scheme where malicious people hide a link in an invisible button on a webpage, and when someone clicks the offending button it executes a line of malicious code that can hijack their browser or PC. They are also speculating that this attack could be caused by malware already on the PCs of users who are experiencing these issues.
The hacking group Anonymous threatened to attack Facebook on November 5th, 2011, but those attacks never came. People originally wanted to point this attack to them, but they have denied any involvement.
Facebook is aware of the issue, and have fixed it for the most part. They are tightening up their security to prevent future attacks of this nature. At this point, users just need to practice general good Internet habits and not click any links if they are unsure of what they do. Also, be careful when randomly clicking around on websites as you never know where malicious people may have hidden unpleasant links.