When it comes to reproducing official documents you have to balance realism with utility. Understandably, the powers that be get a little uncomfortable with folks banging out copies that are a bit *too* good. For that reason these faux stamps of the Great Seal of the United States were designed to look reasonably realistic, but are missing the "tells" a real passport or document seal would have.
Click through for the full sized versions. You can overprint an existing document by sending it through the printer a second time, or use a graphics program with layers and/or transparency to apply them before printing.