I didn't realize they advertised serials, but thinking about it -- of course they did. These are posters for some of the series. For the most part the serials, which were played before the B feature were pretty silly (at least I think they were played before instead of after the B feature). If you look around you can find some of the old serials. In moderation they can be entertaining in their goofiness.
These are examples of posters promoting some of the serials. There are more after the jump, and even more at 100 Years of Movie Posters where I got these samples.