Question about the IRI of a RDF subject from an ODF document

[Svante Schubert <Svante.Schubert@Sun.COM>]

Imagine somebody has downloaded an image from the internet (e.g. Flickr) 
and wants to embed the image into the ODF document. Now he wants to add 
further metadata about this image to his ODF document.
What is the usual RDF way in choosing the RDF subject IRI?
Using the flickr URL or the local path to the image in the document to 
describe the image as RDF subject?

Imagine further the graphic would be embedded into several ODF documents.
If a RDF applications scans the ODF documents, it would 'know' that the 
subjects are the same, if the Flickr URL was being used or the local 
images was set equivalent to a Subject using the Flickr URL.

In other words, does the IRI of a RDF subject representing a 'pointable' 
instance should be the URL pointing to it to describe the entity or some 
common URL to express the similarity?

Bests,
Svante