PG should, perhaps, clarify that Art Nouveau and Art Deco are related but separate classes of images.
Art Nouveau lasted from about 1890-1910. Here are a few examples:
Entrance to the Paris Subway by Iste Praetor – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=23491357
“Peacock Room 1908”
Peacock Room featuring the Princess in the Land of Porcelain painting by James McNeill Whistler
Freer Gallery of Art and Arthur M Sackler Gallery [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)]
Cover of Jugend by Otto Eckmann (1896)
Front cover of Wren’s City Churches by Mackmurdo, a print from The Hobby Horse (England), published by G. Allen with woodcut, letterpress, mezzotint on steel.
Alphonse Mucha, an advertisement for Job cigarette papers
Now a bit of Art Deco:
By Weimer Pursell, silkscreen print by Neely Printing Co., Chicago – This image is available from the United States Library of Congress’s Prints and Photographs divisionunder the digital ID cph.3g11941.This tag does not indicate the copyright status of the attached work. A normal copyright tag is still required. See Commons:Licensing for more information., Public Domain, https://commons.wikimedia.org/w/index.php?curid=12570100
December 1919 Vanity Fair cover by French illustrator Georges Lepape (1887-1971).
Curtiss Flying Service Poster, National Air & Space Museum
Art Deco/Art Nouveau isn’t limited to old works. Here are a couple of images from modern artists at 99Designs who may be available to create cover designs.
“Seductive Vintage Beer Label” ChiChiya at 99Designs https://99designs.com/profiles/3238545
From CIN at 99Designs