Someone asked:
I have a funny question. If all looking at a picture is just lights coming from the screen, then is it really assur to look at it?
Answer:
The Rambam in Hilchus Issurei Biyah (21:2) writes that someone who gazes even at the small finger of a woman with the intention to enjoy it, it is considered as if he looked at her "place of filth". But in the 21:2 he writes, that one can look at a woman to see if she may be fit to be his wife, but only if he does so in a non-promiscuous way. We see here clearly that the difference between what is permitted and what is not permitted has nothing to do with WHAT you are looking at, but rather whether it is for the purpose of gazing with lust, or without.
See also this pamphlet on hilchos shmiras ainayim and technology written by a leading Posek. He brings the Halachos in Shulchan Aruch that it is forbidden to gaze even at the colored clothing of a woman (lest he come to think about her) and the issur of reading romantic novels, etc... So kal vachomer images of actual women!
See also this page: http://www.guardureyes.com/GUE/FAQ/FAQ6.asp
Bottom line: Anything that brings a person to arousal, in any way, is strictly forbidden.
But... the REAL danger of looking at these kind of images is that they feed the lust, the addiction and the disease, more than anything else. Even more than the actual masturbation. They are like a hard drug. Seethis articlefor the effects of porn on the brain.
So it's like asking if drinking alcohol is assur for an alcoholic. Even if it would not be a Halachic problem, for an alcoholic it is much more stringent! It's pikuach nefesh. And Chazal say "Chamirah Sakanta Me'issura - a physical danger is even more stringent than a prohibition". The same goes for these images. Even if they wouldn't be assur, they are a dangerous drug which leads to needing more and more, and ultimately destroys the person's life in this world and the next.