If you know how to do it, then set a range to the location of the image you are removing and then insert an image in the range e.g. the following will replace the first inline image in each header footer with the named image.
Code:
Sub ReplaceImage()
Dim oHeader As HeaderFooter
Dim oRng As Range
For Each oHeader In ActiveDocument.Sections(1).Headers
If oHeader.Exists Then
If oHeader.Range.InlineShapes.Count > 0 Then
Set oRng = oHeader.Range.InlineShapes(1).Range
oRng.Text = ""
oRng.InlineShapes.AddPicture _
FileName:="C:\Path\filename.jpg", _
LinkToFile:=False, _
SaveWithDocument:=True
End If
End If
Next oHeader
lbl_Exit:
Set oHeader = Nothing
Set oRng = Nothing
Exit Sub
End Sub