|Image comes from EQD, so I'm afraid I can't help the flash|
The Brony Thank You Fund have asked that anyone who sees the poster, or who may be able to shed light on its whereabouts, get in touch with them. The phone number they give is 603-513-2383 – obviously this is a US number, so you'll have to add the appropriate prefix if you're calling internationally. Let's hope that, one way or another, this poster can be returned intact.
On a personal note? I'm saddened and disgusted by this despicable act. It really is a low act to stoop to. But please don't let it destroy your faith in our fandom. It doesn't mean the fandom is rotten, just that (like any other large community) it contains a certain number of rotten apples. I hope that the far larger number of good people we have will end up giving this story a happier ending.
Edit: according to Graem in an EQD comment, this is the BronyCon 2012 poster – though obviously most of them aren't signed!