Seeing something like this would be traumatic for a big tough policeman. For a small child it would be even more so. I am sorry you saw this. It is obviously so vivid to you still.
I don't think blocking the memory is the answer. Trying to block emotions or memories can cause other problems, like headaches or breaking out in hives or insomnia. However, some form of therapy can help you process the memory, like Terrie mentioned. Have you ever talked to anyone about it? Can you sit down with a good friend over a cup of coffee or a coke and talk about this and really be heard, by someone who won't say, "Just don't think about it"? It might help to say how you felt. I imagine you were terrified, horrified, and afraid it might happen to you or your mother. You might have felt compelled to look at the awful picture, and then felt guilty because you did. You might have felt relieved that it was not you or your own mother, and then felt guilty because of that. My point is that there are lots of possible strong feelings involved. If you can talk about those feelings, it might help to lessen the intensity of the whole memory.
If there is not someone you can sit down with, you could write here or write in a journal.
By the way, have you read anything by Stephen King? I am not recommending that you do :-) but when he was four years old, he saw his best friend run over by a train. He wet his pants and was mute for an entire day. Now he doesn't even remember it, but his mother has told him about it. I think that has a lot to do with why he writes the stuff he writes. So obviously he was affected by that incident. And we are likewise all affected by the traumas we have gone through.
I think it is a very good thing that you are dealing with this horrific memory. I hope you have someone there who can sit with you and then give you warm hugs, but we are here for you too.