"There a few regulations to which the U.S. Treasury must adhere when redesigning banknotes. The state motto "In God We Trust" must appear on every banknote. The bill requiring this, H.R. 619, was introduced by Representative Charles Edward Bennett of Florida. The inclusion of the motto was meant to serve as a frequent reminder that the country was founded upon faith in God as well as spiritual values. President Dwight D. Eisenhower signed this bill in 1955, but it was not until 1957 that banknotes bearing this motto were first circulated.
The portraits appearing on the U.S. currency can feature only deceased individuals, whose names should be included below each of the portraits. Since the standardization of the bills in 1928, The Department of the Treasury has chosen to feature the same portraits on the bills. These portraits were decided upon in 1929 by a committee appointed by the Treasury. Originally, the committee had decided to feature U.S. Presidents because they were more familiar to the public than other potential candidates. The Treasury altered this decision, however, to include three statesmen who were also well-known to the public: Alexander Hamilton (the first Secretary of the Treasury who appears on the $10 bill), Salmon P. Chase (the Secretary of the Treasury during the American Civil War who appeared on the now uncirculated $10,000 bill), and Benjamin Franklin (a signer of the Declaration of Independence, who appears on the $100 bill).
By the Legal Tender Modernization Act (2001), Congress prohibits any redesign of the $1 bill."
https://en.wikipedia.org/wiki/Federal_Reserve_Note
