Taking a cue from Maverick Collecting's suggestion that press photo's may have influenced space box art I looked into Robert McCall's ROLLOUT above. Painted in 1964 originally for LIFE magazine, it pre-dates the construction of the Vehicle Assembly Biulding (VAB) pictured in it by at least a year. As such it was an artists's impression completely. So I was amazed to find the NASA photograph below. Uncannily similar it pictures Apollo 16 in 1972, eight years after McCall's painting! There will no doubt be many photo's of one of the many Saturn V's emerging from the VAB but this is the only one I could find online.
Pictures: top - Look and Learn under licence, bottom - NASA.