For Personal Journals:
set latest_optout yes
For Communities (Maintainers are the only ones who can do this, I think):
set for "community name" latest_optout yes
In place of 'community name', put the username of the community you maintain and leave the quotes there. Then click "Execute". You can change this later, if you like, but copying the same codes and replacing 'yes' with 'no'. I have already taken care of all of the communities I mod, so if you post icons or pics to the communities and don't fake cut to your own journal (unless you've opted your own journal out), you're covered. It's not retroactive, so if they've already gotten your icons or pictures at iconscraper, there's nothing much you can do. They'll just insult you and make fun of you if you try to ask them to remove it. But at least all future posts will be protected. I'd advise anyone who's worried about this to opt out. It only takes a couple of seconds. It took me less than a minute for all 9 of the communities I run and my own personal journal.
Cross-posted a bunch of places, including my Flist, and thanks to cae_prince over at __nyicons for the heads up.