<Encyclopedia Astronautica didn't list it in their alphabetical categories under either X-33, X-34, or Venturestar. A search at the Boeing Rocketdyne website didn't turn up anything like what you describe either (although they sell some very awesome posters!). If Rocketdyne had the image on their website then the only way it would appear somewhere else is if someone stole the image and saved it on their server. It would be nearly impossible to find someone's personal website that had the image, and news sites like Space.com usually only show the same image as everyone else, never any variety, so the chance they would have it is slim too. I think when the X-33 was cancelled the image galleries ceased to exist, except for a few token pictures kept for posterity.