Paper currency is largely left up to the Secretary of the Treasury. When the bills were standardized in 1929, the Treasury appointed a committee to decide on the portraits. The committee chose U.S. presidents and statesmen because they were more recognizable to the public, and they've been pretty much left alone ever since.In 2016, the Treasury announced several design changes, including replacing Andrew Jackson on the $20 bill with Harriet Tubman. Other changes included portraying Martin Luther King Jr. and women's rights activists Sojourner Truth and Susan B. Anthony, among others. Recently, however, current Treasury Secretary Steven Mnuchin said those changes wouldn't come to fruition until 2030, if at all.