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.