Location: Reina Sofía, Madrid
Picasso's huge mural depicting the 1937 massacre of a Basque village, this painting is controversial for a number of reasons. As a decry against fascism, it's become known as "a picture of all bombed cities." Its home has been the largest point of contention because Picasso did not want it displayed in Spain until justice had been restored there. When it was on display at MoMA in 1967, 400 artists signed a petition directed at Picasso to have the painting removed from the U.S., citing the atrocities of the Vietnam War. In 1974, a young artist named Tony Shafrazi spray painted the words "Kill Lies All" on the canvas. In 2003, a tapestry version of the piece was covered up at the United Nations. Now it is permanently housed in the Museo Nacional Centro de Arte Reina Sofía in Spain.