In the first dot-com boom, I worked on a large groups application, kind of like what Google Groups is now. We had ~3m users, uploading thousands of images per day. For the first 6 months or so, it was the developers who had to do the moderation. We saw a lot of stuff that we could (and, frankly, had to) laugh about - anatomically impressive feats of stretching, comically ludicrous insertions, etc - but then there was the other stuff, the ones that you just couldn't laugh off. Stuff being done to others who clearly weren't old enough to consent. Some of the things I saw cannot be unseen or forgotten, however much I've wanted to in the ten years or so since.
After a while it does get you down. The very ordinariness of the backdrops was what got to me. People's ironing boards in the background. Their work uniforms hanging on the back of the door. You realise that this kind of shit is not done by crazed inbreds in the mountains or by foaming-at-the-mouth psychos, but by everyday people like the ones you sit next to on the bus or who smile at you as you buy a coffee from them every day. And that really got to me. I started looking at people and society very differently, and feeling constantly angry or sad.
In the end we hired a team of dedicated moderators, who had an enforced 1-to-1 counselling session every week. We also started working with law enforcement and people in suits whose cards just listed their job as 'the home office', and every now and again we'd get an email from the higher-ups telling us that our evidence had been crucial in securing a conviction in some case that had been in the news recently. And that helped.
There are far worse things on the internet than Goatse or tub girl, and a depressingly large number of people who produce them, consume them, and share them with others. Anyone who does that job for a sustained period has not only my sympathies, but my thanks