They were "work for hire" in the US. That leaves them under US copyright until 95 years after they were printed. 1933 + 95 = 2028 for the first cover.
The Berne convention is life + 50 (stretch by many countries to life + 70) for the artist. Since you know the name, you can look up the artist's date of death and find out when he was to go PD in those countries.
Some of us (such as me) think that copyright length is ridiculous, but we don't link to pirate sites here. This is too useful of a site to have legal problem with it....