For anyone on the lookout for photographs of German composers: I have bought a copy of "Deutsche Tonkünstler und Musiker in Wort und Bild" by Friedrich Jansa, second edition, 1911.
This contains a large number of short biographies of composers and musicians, many of which come with a photograph.
I am happy to scan the relevant entry for anyone who is looking for a composer who is listed in the book and has an entry with a portrait.
Thanks for this kind offer!