The artwork on Prada's walls are works by James Jean. His art was an inspiration for one of Prada's collections and he has been commissioned by her as well. The works really helped me pick the textures and lighting for the space. More works by him can be seen at http://www.jamesjean.com/
The two paintings in Obama's office are the paintings he selected for the Oval office.
Fifth Avenue in the Rain by Childe Hassam
The Statue of Liberty by Norman Rockwell