Try this code - the macro is called ReplaceImages. It is written to run from the current selection point to the end of the document and continuously open the usual dialog until it runs out of pictures in the document.
It doesn't include a method of breaking the code if choose to stop before reaching the end of the document. The logical way to do that would be to detect if a dialog was dismissed by pressing Cancel but I'm not sure how I would do that. Someone else might have a good idea on how to do that.
Code:
Sub ReplaceImages()
Dim i As Integer, aRng As Range
Set aRng = Selection.Range
aRng.End = ActiveDocument.Range.End
For i = 1 To aRng.InlineShapes.Count
aRng.InlineShapes(i).Select
funReplaceInlineShape aRng.InlineShapes(i)
Next i
End Sub
Function funReplaceInlineShape(origShp As InlineShape) As Boolean
Select Case origShp.Type
Case msoAutoShape, msoFreeform, 6 'this is a shape with a picture fill
CommandBars.ExecuteMso ("ObjectPictureFill")
Case wdInlineShapePicture 'this is a picture placed into the doc
If CInt(Application.Version) < 16 Then 'following only works pre Word 2016+
CommandBars.ExecuteMso ("PictureChange")
Else 'following is for Word 2016+
CommandBars.ExecuteMso ("PictureChangeFromFile")
End If
End Select
End Function