Prior to Photoshop, photo manipulation was a much more difficult process than it is today. Artists would often have to use photographs as the framework of a project to create a more colorful and eye-catching image.
In the 1940s, the process of painting photographs led to the often ignored commercial art form of pin-up, which featured painted or drawn illustrations of suggestively posed women who were usually only partially clothed. The art form gained its name from World War II soldiers who would pin the illustrations up on their walls while stationed away from home.
Pin-up girls were often told to portray the look of innocence, almost like a pouty “oops” expression, and the “duck face” was born.