Hi there,
I’ve never entered a photo contest on VF before. The topic is interesting but quite niche, as there can be some confusion. It’s sometimes tricky to tell Art Nouveau and Art Deco apart, since overlaps aren’t uncommon. That seems to be the case with the example of Cartagena’s city hall you mentioned—it strikes me as more of an eclectic (or historicist) style from the very early 20th century. Plus, the four pillars in the entrance hall look more Art Deco-inspired to me.
That said, you’re right to point out that the two styles have nothing in common. It’s a bit like confusing the classicism of Versailles or Saint-Sulpice with the baroque of Rome’s Church of Saint Ignatius of Loyola. Art Nouveau is a world of flowing lines, abundant plant motifs—especially floral—arabesques, and pastel tones.
See here: Art Nouveau in Riga (Latvia)
Art Deco, on the other hand, is defined by geometric rigor, straight and clean lines, and a return to symmetry, which is absent in Art Nouveau. Both styles can feature human figures, but they’re more stylized in Art Deco.
See here: Art Deco in Napier (New Zealand)
Art Nouveau buildings and works of art aren’t hard to find—and not just in Europe. Sometimes, all you have to do is look up... but do you always have a camera on hand?
These masterpieces can be right in your own backyard:
Here in Troyes (1908)

Or in a museum, like this stained-glass window in Limoges by master glassmaker Francis Chigot, *The Bathers* (pre-1920).
(But is this really a photo? It’s more of a reproduction, a photographic document.)

Time to dig into your photo archives! ๐