Lindsay,

I have done something similar by creating a MailMerge Main document into which you add an IncludePicture (Insert->Picture->From file) then change the filename to be your map image MergeField.

{ INCLUDEPICTURE "{ MERGEFIELD map }" \* MERGEFORMAT \d }

After the MailMerge has run select the entire document and perform a

.Selection.Fields.Update()

to update the images.
HTH