You could insert the picture as linked image into the document template; that way it's available to all documents based on the template, without the need for any of them to actually contain the image. A possible advantage is that, if you decide to change the image, changing the one linked file will update all the documents linked to it; in some situations that might be a disadvantage, so use wisely.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]