You both make good points, Chrome and TBS. One principle I'm trying to emphasize is that, with anything, be it treatment, medication, or risk factor, there are going to be people who are going to have a certain outcome regardless, and those to whom it will make a difference. Yes, there are going to be creeps who go crazy no matter what you do to either prevent or cause it, and there are those who will not, no matter what amount of crazy you throw at them. What you want to look at is how many people will it actually push over the edge? That's the vulnerable group. Again, there are hundreds of different factors at work, so it's all but impossible to point at one and say "See, that's what did it!" But repeated exposure to messages can have lasting effects. Heck, in a single experimental session, women can be induced to respond less favorably on body- and self- satisfaction surveys just by having them look at a few fashion models. As has already been pointed out, if media didn't influence the way we think, it would have gone out of business ages ago.
Yes, violence and whatnot have been around for ages. What media imagery can do is exacerbate the situation. Again, you look at the vulnerable group. I work on a daily basis with people who have been on the short end of this sort of thing, and it ain't pretty. You try working with someone who's been a victim of rape, or systematic physical abuse, or any number of other ills, and see how your views change. These problems will never go away, but there are things we can to do reduce them. Trouble is that there's too much money in them, so no one wants to hear it.