No one can truly know what another person saw during a period of time in which they were clinically dead. It's easily possible that these images are nothing but the brain's last thoughts, but whether it's an afterlife is not for us to judge.
It could simply be the brain playing a trick on the person... But, if they died, then how would they be able to tell anyone about it? If they were clinically dead, then suddenly regained consciousness (apparently this has happened before, though I have no idea if the information is true or not), then it be prudent to remember that they may be incredibly confused, or they may be seeing what they think is the afterlife, even if it isn't. All-in-all, it is hard to know.
