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HankronI've never owned one before, and I'd have to see one in person to tell you for sure.
My guess is that they are halftone photoengravings (photoengraving being a popular printing rival to lithograph) and are not real photographs like many think. If under strong magnification they have the halftone dot pattern, that's what it is. If this is the case, this would mean the images are reproductions of original photographs or whatever the original art was (some of them have a kind of artisticly-altered look). This is the same printing method used to make many 20th cards, including the Sporting News, Sporting Lifes and Exhibit cards. These all kind of look like photographs but, upon closer look, have the dot pattern.
Again, I'd have to see one in person to say for sure.