Thanks, I've given it a try. Well, this is interesting.

I connected the file to the data source. For general use, it's fine; the images of signatures appeared as expected. I then tried two versions of emailing:
For the first I simply saved and emailed myself a copy of the file without any other changes.
For the second, I ran Finish & Merge on the current record, then emailed it to myself.

Both versions display the signatures I expected. However, it's still looking in the M drive location. If I rename the file or otherwise make it unavailable, the link breaks and the image doesn't display.

I've tried both .doc and .docx versions to make sure it's not a compatibility issue, but no joy.