is there any incentive to participate in your community?
With moderators you feed the "power tripper".
With karma you feed people obsessed with points.
This is a bit complicated: what if you had some sort of capcha that required users to classify images as nsfw/sfw/illegal?
If the user normally encounters photos on the site by requesting them (e.g., by entering a search query or browsing a friend's album) rather than having random photos thrown in their face (like HotOrNot.com), I would think you could run into some very upset users (and possibly legal problems) if you are throwing random photos that might contain disturbing images in their faces. I mean, if you go to a website intending to browse photos your friend took of his boat and the site throws up some random child porn on your screen, you'd be pretty annoyed, right?
prompt a capcha when someone attempts to `upload` a file
I do support Twitter login which is fairly common, and have had a few hundred users sign up (out of like >10mil uniques), but I wouldn't say that they've exhibited overly engaged behavior after doing so. They're basically the same from a stats perspective from what I've observed.
I'm afraid to make user logins compulsory, especially considering the kinds of knuckleheads that are on my site.
How do I get from "eh, have fun as a guest" to "everyone's got an id" without destroying my stats?
I also question the assumption that taking steps toward accountability and community will hurt the numbers. Maybe in the short term, but if your site becomes a cesspool it will eventually drive your real users away anyway. If you figure out how to sustainably handle the filth, it may be possible to attract a much larger audience who would otherwise have been repulsed. Yik Yak is an example of that strategy.