I'm trying to assemble a large document, and many of the included
(sub-)documents refer to the same illustrations. The illustrations all
go into one file of their own.

This results in several advantages: one image can be repeatedly cited
and still easily found; the whole run of illustrations can be printed on
a different paper stock; it eliminates problems from Word's shoddy text
flow algorithms that routinely ram illustrations into the margins and
separate them from their captions.

The question is, how should I insert the cross-references?