The Bildarchiv Austria is the digital image collection of the Österreichische Nationalbibliothek in Vienna with some 220,000 digitized images, including those from the Graphiksammlung. Among the collections relevant for the First World War are the archives of Austrian photographers. Information about some of them is given in introductory guides with a selection of their work. You can use a simple search mode (Schnellsuche) or the advanced search mode (Profisuche). The Bildarchiv Austria gives you also links to other image collections. The search interface is in German.