Having seen the document (or rather not seen it as the linked images are not available to the forum) You will have to delete and re-apply the images, and frankly as the images are not included in the document, their file sizes shouldn't be an issue as they are called from file to display them. However, the following macro should do the job. Change the portrait and landscape image file paths as appropriate. The macro assumes that there are only two images used (as seems to be the case in your document). If there are more then you will have to add them and conditionally insert them according to section, much as I have shown with portrait and landscape.
Sub ReplacePicture()
'Graham Mayor - http://www.gmayor.com - Last updated - 21 Jul 2017
Dim oSection As Section
Dim oHeader As HeaderFooter
Dim oShape As Shape
Dim t As Long, l As Long, h As Long, w As Long
Dim sName As String, sPicture As String
Const strPathP As String = "C:\Path\PortraitPicture.jpg"
Const strPathL As String = "C:\Path\LandscapePicture.jpg"
For Each oSection In ActiveDocument.Sections
For Each oHeader In oSection.Headers
If oHeader.Exists Then
For Each oShape In oHeader.Shapes
With oShape
t = .Top
l = .Left
h = .Height
w = .Width
sName = .Name
End With
oShape.Delete
If oSection.PageSetup.Orientation = wdOrientLandscape Then
sPicture = strPathL
Else
sPicture = strPathP
End If
Set oShape = oHeader.Shapes.AddPicture(FileName:=sPicture, _
LinkToFile:=msoTrue, _
SaveWithDocument:=msoFalse, _
Left:=l, _
Top:=t, _
Width:=w, _
Height:=h, _
Anchor:=oHeader.Range)
oShape.Name = sName
Next oShape
End If
Next oHeader
Next oSection
lbl_Exit:
Set oShape = Nothing
Set oHeader = Nothing
Set oSection = Nothing
Exit Sub
End Sub