If you have linked the image, then there are actually two approaches:
1. Say your linked file is called picture.jpg. Remove the original
picture.jpg and replace it with another of the same name. Word will now link
to this file instead.
2. If the linked graphic is In Line With Text, pressing Alt+F9 will expose
the field code, which will be something like this:
{ INCLUDEPICTURE "C:\\Documents and Settings\\UserName\\My Documents\\My
Pictures\\picture.jpg" \* MERGEFORMAT \d }
Say you want to replace picture.jpg with photo.jpg, which is in the same
folder. Use a batch Find and Replace to find INCLUDEPICTURE "C:\\Documents
and Settings\\UserName\\My Documents\\My Pictures\\picture.jpg" and replace
it with INCLUDEPICTURE "C:\\Documents and Settings\\UserName\\My
Documents\\My Pictures\\photo.jpg". Of course you could use any portion of
that field code that is unique, and if the path is different, you could
substitute the new path. For batch F&R, see
http://word.mvps.org/FAQs/MacrosVBA/BatchFR.htm or
http://www.gmayor.com/batch_replace.htm