Magic Lantern slides were shown in theaters to advertise coming attractions. They also had another use for them.
This turned up on ebay recently:
Magic Lantern Glass Slide
THIS SLIDE was used as a "Can you name the actors?" type of exercise to keep the audience entertained while waiting for a movie to start. (This same type of game still happens in some movie theaters today - although with more recent stars). With a bit of research, movie buffs may be able to figure our which movie(s) the movie stills are from.
The Thelma Todd and Patsy Kelly picture is from AIR FRIGHT. I don't know about the Dorothy Stickney picture.