Curious Frau has a well-done guide on using the search engine at bildindex for non-German-speakers. With a link to a bildindex tutorial with screenshots by saragrace.
Both of these persons seem to be costume designers.
Curious Frau's main page, while not having been updated since last August, nonetheless features a tutorial/guide to the search engine at Der Institut für mittelalterliche Realienkunde / Digitale Bildsammlun, from whence the following detail and click-through seasonal image.
Maria mit Kind (detail)
Tomaso da Modena