7/4 already answered the question... it came out of a whole series of photographs that Stock took of Dean just before East of Eden opened in March of 1955. If you've ever seen the Fairmount, IN farm shots, including the famous "You can't go home again" photo (where Dean is standing in front of his uncle's house, looking to his left while his dog turns to the right), those come from the same series. Stock just published a book of those photos:
Stock, by the way, did a book of jazz photography called JAZZ STREET.
I devoted last week's Night Lights episode to Dean and Robert Altman's 1957 documentary The James Dean Story. I actually thought about driving up to Fairmount today... I was up there with my mom once before, when I was about 19, and we went to the gravesite. There were thousands of people up there last weekend for the annual Museum Days; Dean still exerts a powerful pull, evidently.