Gil Elvgren (1914-1980) painted calendar girls and commercial illustrations from the 1930s to the 1970s.
Here are some of his oil paintings paired up with the photo reference. He makes a thousand little drawing refinements, such as a thinner waist and more tilt to the back. The tones of the picture are simplified, and he uses the Windmill Principle. He's not afraid to let the shorts be white on white, and the legs be red on red.
From Retronaut courtesy of Chris Wild
Lots more Pin-up/ Photo pairings at a Concept Art forum (Thanks, Brian Bridges)
Gil Elvgren on Wikipedia
Post about Elvgren's process on the Underpaintings blog (Thanks, Tomas)
Gil Elvgren website
Book: Gil Elvgren: All His Glamorous American Pin-Ups
Previously: Using Photo Reference (with 32 comments)