While I am very saddened that a dog may have been killed, unless these guys were tried and convicted, this is not an expression of free speech! This could be defamation and you could be held accountable simply for sharing the image on your website.
You don't know the story behind this tragedy or if a tragedy even occurred. For all you know, these two guys broke some girls heart and this is her way of getting revenge. That probably isn't the case, but do you want to risk being a part of that?
Free speech does not give anybody the right to say whatever they want. With free speech comes responsibility. You have the responsibility to be truthful when writing about other people. This goes for FaceBook, chat forums, books, newspapers, blogs, etc.
I see it happen here quite often. Somebody buys something and ends up coming to the forum to say they were "scammed" or the seller is a "thief", etc, etc. These are the types of things that can get you sued. There are dozens of cases where people have sued for much less and got awarded big money by the courts.
I know some of you will disagree and are thinking you have the right to post whatever you feel. This saddens me, because it is this kind of thinking that will eventually lead to overzealous lawmakers restricting all speech online.
Be smart with your words and keep yourself out of court.