I would think the simplest way to do it is to have [[image:...]] reference the English wiki, and [[bild:...]] the German one.
So what do you call the French one? And isn't "bild" also in some other Germanic language?
The latter isn't a problem; it can link to local images in the Dutch wikipedia as well--I didn't intend to be able to cross-link from any language to any other, just to have "local" and "main" images. But your point about French is a good one. I aleady do accept links like "en:user:Fred", so "en:bild:Fred.jpg" isn't much of a stretch.