I've never heard of movie lobby cards, but do remember that 25 cents used to buy a candy bar, a coke, and a ticket to the Saturday Matinee at the local movie theater. Got a nickle back in change too.
These original lobby cards, which are really works of art in their own, were apparently a big thing back when a theater lobby was a lot more formal. They appear to be of various mediums: B&W photographs, lithographs based on movie stills, offset B&W lithographs that look to be colored at the lith stage, etc.
I wanted to like this article, but it may need fact checking. It mentioned that 90% of silent films were lost because they were on silver nitrate........ but that's just the conservator's guess. Who, by the way, is also a realtor and historic preservationists, so those figures are, well, just figures.
A quick google search showed the amount of lost films were estimated to be from 70% to 90% (on the first page), and not all silent films were on silver nitrate. I guess they're scanning everything, some of the cards are over a century old. Hopefully that's not Realtor Speak for "They look old. Let's say they're a century old, just to be sure."
More than 10,000 lobby cards once hung in movie theater foyers are now being digitized for preservation and publication, thanks to an agreement between a collector and Dartmouth College