I agree with Chad's answer about not saying anything negative. I also think it's important to be honest in interviews, as failure to do so could cause problems later (worst case: termination of the job, although I think that would be highly unlikely in your case as it's described). I know it's tough when these two objectives seem to be in conflict. That's where answers like "I am looking for a position with more opportunity for growth" are good.
In the case where dealing with disturbing content is a motivation to seek another job, using an answer such as "I am looking for a position with more opportunity for growth" is OK, but I do think that a more detailed explanation might be good, maybe phrased as:
Currently, I am engaged in $legal_and_legitimate_activity which exposes me to intense media content. I am starting to find this content is making me feel upset/disturbed/unsettled and am seeking to transition to a similar role that does not deal in such content.
If you do this, I think it's very important to stress that the activity that exposes you to the content is completely legal and legitimate. Working with a system that files/manages media crime scene photos, or images of medical conditions (sounds close to what you are working with), or journalists' photos of conflict zones, or building an automated image filtering/moderating system for web forums would all expose you to such content and I don't think that would cause anyone to be suspicious of you or to think negatively of your former employer.
If you can't think of a single explanation that doesn't seem shady, you probably shouldn't mention this aspect at all, and stick with the "growth opportunities" answer.
In the case that any new employer would work with such content, it would probably be good for the interviewer to know you no longer want to work with such content as soon as possible, rather than for you to find out the details on your first day on the job. I think this can be generalized to any situation that causes you to move jobs but does not reflect poorly on you or the previous employer.
Also, Juha's answer makes a good point: have you spoken to your current employer about how this work is affecting you? Even if they have no other assignments, it's probably something they should mention when they interview new hires. If the problem is really bad, would they help pay for therapy? I remember reading a story about someone audited objectionable content with Facebook and they ended up paying for his therapy, as the stuff he saw affected him that deeply.