I've tried to strike a balance between shrinking the files to manageable size and still making them large enough to be readable, but the original magazines are huge and the pix are just plain BIG.
My vintage magazine collection is small and haphazard. The dates are unevenly distributed, with big gaps. And at least for now, I'm just posting for fun and have done no research about any of this. So they in NO way provide "proof" of anything about the history of ads, but interesting observations stand out.
Ads in the style of a comic strip really don't exist until the Depression is well underway. The biggest stack I've got is 1920's magazines, and there's not a comic strip ad in any of them.
Modern ad methods are starting to appear. I love both of these: the graceful lines and the use of white space in the Lifebuoy ad (McCalls, Aug. 1926); and in the Atlanta Chamber of Commerce ad (Time, Sept 10, 1928), the energetic, stylized, rather jagged images that make me a little antsy, as an evocation of the "impatient age" should. Cartoon drawings abound, but not strips with word balloons that tell a story.
In the early 1930's, comic-strip ads appear and then they really seem to proliferate. Through the '30's (and '40's) there are several in every magazine. My Deep Thought Of The Day is that tougher competition for customers spurs innovative advertising methods. Advertising may be an industry that's valued more than ever during hard times, and primed by the economic-stress pump.
The earliest in my collection, come from Woman's Home Companion, November 1931. Two comic strips ads nearly identical in format and page placement. Not really terribly interesting as ads go, but the form was new and different!