Anyone who has access to the file could extract the jpg image and replace it with a tiff version - and all you'd need for that is to open the jpg image in any image editor and save it in the tiff format before reinserting it into the document. Regardless, Word's document compare utility shows that the image was replaced, and digging into the file data is how I found the details of that change.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]