private void button1_Click(object sender, EventArgs e)
{
    Microsoft.Office.Interop.Word.Application worddoc = new
    Microsoft.Office.Interop.Word.Application();
    worddoc.Visible = false;
    object docpath = "c:\test.docx";
    object objYes = true;
    object objNo = false;
    object defaultValue = System.Reflection.Missing.Value;
    Document myDoc = worddoc.Documents.Open(ref docpath, ref objNo, ref objNo, ref objNo,
                                            ref defaultValue,
                                            ref defaultValue, ref defaultValue, ref defaultValue,
                                            ref defaultValue, ref defaultValue, ref defaultValue,
                                            ref defaultValue, ref defaultValue, ref defaultValue,
                                            ref defaultValue, ref defaultValue);
    if (myDoc.Sections.Count > 0)
    {
    myDoc.Sections[1].Headers[WdHeaderFooterIndex.wdHeaderFooterPrimary].Shapes.AddPicture("c:\yay.jpg",
        ref defaultValue, ref defaultValue, ref defaultValue, ref defaultValue,
        ref defaultValue, ref defaultValue, ref defaultValue);
        //^this adds an image to the header.
        foreach (Section section in myDoc.Sections)
        {
            Range headerRange= Section.Headers[WdHeaderFooterIndex.wdHeaderFooterPrimary].Range;
            headerRange.Fields.Add(headerRange, WdFieldType.wdFieldPage);
            headerRange.ParagraphFormat.Alignment = WdParagraphAlignment.wdAlignParagraphRight;
            //^sets alignment to the right, this works fine.
        }
        myDoc.Sections[1].Headers[WdHeaderFooterIndex.wdHeaderFooterPrimary].Range.Text="myTex";
        //^this adds my text to the header, but it overwrites the picture, thus deleting it.
        }
    }
}