In my humble opinion the origin of both Photoshop AND glamour photography falls smack in the lap of George Hurrell.
He was the photographer of the stars in the "heydays" of Hollywood (1930's).
I am not going to spend too much time today talking about Mr. Hurrell, but will leave you with enough resources below so you can read to your hearts content about him, his style, and gaze at his work.
If you had to pick out the two things that really separated George from the pack it was his lighting style and editing technique.
He would ask his stars to wear little to no makeup which made it easier for him to actually retouch the 8x10 negative produced by his large format camera (using things as simple as a pencil).
Here is the original photograph of Joan Crawford on the left next to the same photo on the right after the negative had been retouched:
More glamour shots of Hollywood stars photographed (and edited) by George Hurrell, he often lit so that the subjects eyelashes would cast dramatic shadows (click on any image to view larger):
Here are some links you will want to check out:
Hurrell Estate Editions
From The Atlantic "Starlight and Shadow"
The Telegraph "George Hurrell: the master of the Hollywood still"
The Reel Foto "George Hurrell: Lights, Camera, Glamour!"
Watch the 1 hour documentary "George Hurrell Documentary "Legends in Light"
Here is a video the emulates the classic Hollywood glamor, but using todays tools and techniques "Classic Hollywood Glamour in Photoshop"