If you want to delete the bookmark, you need to add code to do just that. For example:
Code:
With ActiveDocument
If .Bookmarks.Exists("Sig1") = True Then
With .Bookmarks("Sig1")
.Range.InlineShapes.AddPicture FileName:="C\Sig1.png"
.Delete
End With
ElseIf .Bookmarks.Exists("Sig2") = True Then
With .Bookmarks("Sig2")
.Range.InlineShapes.AddPicture FileName:="C\Sig2.png"
.Delete
End With
End If
End With