Best way to represent an image placeholder?

Hi,

We're currently using DocBook 5.0 to markup copies of some
historical printed publications.

These publications can be quite complex and often contain
diagrams, maps, charts and figures etc.

We don't currently have images for these page graphics but
I'd still like the markup to contain some kind of "placeholder"
that indicates a missing image.

My tests with <imageobject> and <imagedata> show I could
use a dummy "fileref" attribute, but that doesn't seem
right.

Can someone suggest a better approach?  Should I be using
a processing instruction or some kind of parenthetical
note?

Any advice much appreciated,

Stu

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Stuart Hungerford
ANUSF Data Intensive Projects