As early as 1898, Picasso was interested in inverted imagery, as hinted at in a drawing known as 'Un Sabio' (One who knows) in which a depicted connoisseur is closely examining an inverted picture of a nude. In Picasso's Cubist art he experimented with concealed portraits of harlequins and other devices, much of it related to mythology and the occult. By the mid 1920's hidden imagery had become an established part of Picasso's repertoire. Because of his exceptional skill at concealment, this aspect of Picasso's art has been almost completely disregarded by historians and biographers.
In The Unknown Masterpiece, Picasso has used almost every line and brushstroke in a intentional way. Even the apparently random flow of ink wash has been used to conceal meaningful imagery. In both Guernica and The Three Dancers, Picasso employed a similar technique to achieve multiple layers of hidden imagery. Because of the concentration of hidden imagery in The Unknown Masterpiece 1934, it provides a key to identifying similar imagery in other works as well as a new way of understanding the hidden themes in Picasso's art.
Melvin E. Becraft's Picasso's Guernica - Images Within Images, 2d Edition, 1987
Langston, Linda Jane Frank, Disguised Double Portraits in Picasso's Work, 1925-1962, Phd Stanford University Phd, 1977. UMI.