I usually add the German keywords. Do you still look over these images and add additional German keywords to the images? Because I am not use to keyword my images in German, because I can see that it is problematic if the images do not have German keyword. I think Zoonar generates the most sales through images that were found with German keywords.
You have a very interesting post on your German blog
http://www.zoonar.de/blog/2009/11/30/zoonar-bearbeitet-fotos-bis-grose-a1/ about Zoonar's service, that I think is very unique. It is good to know that you offer an excellent service to buyers too (conversion A4 (similar to letter size) to the size of A1 (23.4 in 33.1 in))