That's not the way this would be prosecuted. It's one thing to have a picture; it's another entirely to plaster it all over the side of a bank. One is obscenity; the other is criminal defacement and possible restraint of trade.
Criminal defacement might be a winner, but since FR does not engage in trade (we explicitly say so on the front page), restraint of trade wouldn't be applicable.
Not to mention the fact that he's pinging threats (Jews and N*****rs should be lynched) to specific FReepers.
I haven't researched the issue in depth, but at a cursory analysis, I think the First Amendment would make any such criminal prosecution theory sink...and you would find it difficult (to say the least) to get a U.S. Attorney to even look at the case (based on a theory of obscenity and interstate trafficking in porn). We weren't able to even get the ban on kiddie internet porn to withstand the absolutists currently haunting the U.S. Supremes.
And unless the perpetrator just so happens to live in your jurisdiction, you will not be able to enlist State or County agencies...which would be even less likely to do anything. Unless you could first obtain a protective order (TRO in legal parlance) to bar the nuisance activity. Then repeated violations would give you more solid standing to go after the character.
The victims and the Corporation (Free Republic.com) would perhaps have standing to bring an civil action sounding in tort law, the tortious interference in business (restraint of trade), defamatory damaging of business reputation, and on the personal level, intentional infliction of emotional distress.
As a furst step, I suggest that the Moderators be given the ability to "nuke" all posts and email by such an obvious troll...and spare the victims the angst that you rightly express. I assume that the guy must be using an alias when he registers.
Perhaps a precondition should be made so that Free Republic posting privileges require a secure ISP-tracking cookie be enabled on the posting system.