Some images should be on a wiki in a particular
language (for
instance the map of China and environs) and some should be on
one wiki and usable by all (most chemical diagrams, pictures
without words). How about [[en:image:foo.jpg]] to mean that an
image is on the English wiki, even if the link is on the German
one?
I would generally prefer to encourage duplication; that way, the
whole German (or Spanish, etc.) wiki could be easily reproduced
from a database and image directory backup. But since images are
kind of "external" anyway, and the links properly define where to
get them, I might be convinced that inter-wiki image links are OK.
I would think the simplest way to do it is to have [[image:...]]
reference the English wiki, and [[bild:...]] the German one.