The digital collection Die historische Kriegssammlung der Hofbibliothek 1914-1918 is a project of the Österreichische Nationalbibliothek in Vienna. The collection presents a wide variety of resources: posters, pamphlets, drawings and engravings, albums, postcards, children’s school essays and much more. Interestingly this collection was created already during the First World War at the former Hofbibliothek.
Some of these collections can conveniently be searched using the interface of the Bildindex der Kunst und Literatur, Marburg. The search interface is in German. The postcards can be found by choosing in the Gesamtindex for “Gattung” and entering “Postkarte”.