These were often "photo-shopped". Painted either directly over photos, like the one above, or the artist drew on a transparent layer. but regardless, the quality of the advertisement illustration of the era was extremely high. The lineart illustrations were fantastic. 70's saw some of the best ad art I've seen and then it all abruptly ended.
Is that because back then colour photography was still too expensive?
A quick google tells me colour photography became readily available in the 60s but that the prices remained high untill the 70s.
I assume that in the late 40s it was probably cheaper/easier to take a B&W pic and have it colorized by hand than to take a colour picture.