Photos, when they have been properly altered & captioned, or are accompanied by offensive prose, can offend in a manner most swift and fastidious. You begin by finding something about somebody's behaviour that pisses you off. You can take a screen shot which exhibits their stupidity and make it more explicit to them by adding simple text to the image. Example:
I was looking for artistic depictions of syringes and intra-venus drug use on deviantART. Instead I came across a bunch of douchebags whose only artistic intention was to appear morbid. I wanted to alert them of the fact that they were douchebags, since no doubt their own friends kept silent about the crummy nature of their "photography", so I made a simple little picture:

Alone, this is insufficient to fascilitate any kind of self-realisation by such people. So I let them in on MY artistic intention with a brief summary of their obvious inadequacies. I mentioned the purpose of art (to convey something of meaning) and that appearing morbid does not fall under this category. I gave helpful (condescending) tips to encourage less lameness on their behalf in the future. I suggested they avoid making "art" about subjects entirely foreign to them, or at least research injecting before including it in a self proclaimed work of art. Pointed out that if they are simply positioning what looks like a syringe at a 90 degree angle to their arm and pouting, it is obvious that their intentions are to create a new breed of morbid coolness and not to communicate anything (because clearly they know nothing of Sir Inge).
An effective slaying, one with the potential to really change a particular stupid human behaviour. The key is to find logical arguments, and never resort simply swearing at them; despite how tempted you may feel to simply abuse them. Slaying will not only hurt their feelings, it may discourage further stupidity, which is what we all really want. Give obvious examples of their mistakes in a self-confident tone and generally they will listen. They're sheep, they do stupid things because they are misguided sheep. Show them the way, then hit them with your staff a few times to show them who's boss, and also ensure that they vividly remember the direction to flock to.
Try it, encourage people to change by rubbing their crummy behaviour in their faces. Effective and easy, and they may tell their friends to also stop doing stupid things - the ripple effect of your slayings can be huge. Also, it's good fun.