Most people think that the pin-up images and illustrations we see today were drawn from memory. In fact, most were drawn using real posed photos of models in studios that were later drawn into their final pin-up scene. Marilyn Monroe got her start as a model doing such work, which didn't pay much but was still work.
Enjoy this montage of photos vs. final drawings. Some of the photos actually look pretty enough to stand on their own as final pin-up renditions.
All images by Gil Elvgren.