US Presidential Seal — Display of any likeness of the US Presidential Seal is restricted by US Federal law under 18 USC 713; however, use in encyclopedias "incident to a description or history of seals, coats of arms, heraldry, or the Presidency or Vice Presidency" is allowed under Executive Order 11649.
Photo Credit: Unknown - Extracted from the title page of PDF document at http://www.whitehouse.gov/nsc/nss.pdf