For a primer on inconography, start with the Grove Online entry.
A more indepth study is available here: Studies in Iconology: Humanistic Themes in the Art of the Renaissance (Erwin Panofsky)
To search library catalogs for images of a performance venue, do a subject search for the name of the venue as listed in the Library of Congress’s authorities database. For example, search “Burgtheater (Vienna, Austria)” and “buildings, structures, etc.” in WorldCat to find books about this venue, which may (or may not) include images. For example.
Performer (ML416-429) and composer (ML410) biographies often include images, and instrument-specific reference works typically include technical diagrams of instruments, as well as pictures of performance.
ARTStor - a searchable library database of images and artifacts
Bildarchiv und Grafiksammlung - image archives at the Österreichische Nationalbibliothek
Dayton C. Miller Musical Iconography Collection - digital images from the collections of The Library of Congress
Early Music Sources - Includes treatises, inconography, and a variety of other sources.
Répertoire International d'Iconographie Musicale - international, searchable database of musical images
Music Iconography at Acadia University - a list of online sources related to music iconography