Guernica was a painting by Pablo Picasso. It showed the destruction of Guernica at the hands of the Luftwaffe in 1937. Guernica was commissioned by the Republican government for the Spanish pavilion at the 1937 World's Fair in Paris.
Fitz Kreiner carried a reproduction with him while visiting Paris in 1937. He noticed that it seemed to have more of an impact as the perception of history had been altered. Therefore, he, the Eighth Doctor and Anji Kapoor found the reproduction to have more of an impact than the real painting when they went to see it in a gallery. (PROSE: History 101)