How to: Change a Picture's Z-Order and Text Wrapping
The code snippet below uses the Shape.ZOrder property to position an image behind the specified drawing object and sets the Shape.TextWrapping property to place the image behind the text.
Document document = server.Document;
document.LoadDocument("Documents\\Grimm.docx", DocumentFormat.OpenXml);
Shape myPicture = document.Shapes[1];
myPicture.VerticalAlignment = ShapeVerticalAlignment.Top;
myPicture.ZOrder = document.Shapes[0].ZOrder - 1;
myPicture.TextWrapping = TextWrappingType.BehindText;