In basic form:
Sub ChangePic()
Dim oFile
oFile = Dir("C:\Users\Maxey\Desktop\BatchFolder\*.doc*") 'Change this path to reflect your batch folder path.
Do While oFile <> ""
Application.Documents.Open oFile
With ActiveDocument.StoryRanges(wdPrimaryHeaderStory)
.GoTo what:=wdGoToGraphic, Count:=1
.Delete
.InlineShapes.AddPicture FileName:="D:\Replacement Pic.jpg" 'Change this file path and name to reflect your new picture path and name.
End With
If ActiveDocument.Saved = False Then ActiveDocument.Save
ActiveDocument.Close
oFile = Dir
Loop
End Sub