The comments here are telling. LiveJournal, using typical "due to positive response to our announcement!" corporatebabble, claims people were clamoring for this. No one can find any instance of positive user response. So someone else must have been responding positively. Users be damned. Anyway, here's the official way to disable them:

Go to http://www.livejournal.com/manage/settings/
UNCLICK Graphic Previews - seems to be the default personal journal setting thankfully, though it seems to default to ON in community posts.