I use the following code to add a Picture but i dont adjust image size
var myImageFullPath = "ex.jpg";
using (DocX document = DocX.Create("ex.docx"))
{
    Image image = document.AddImage(myImageFullPath);
    Picture picture = image.CreatePicture();
    Paragraph title = document.InsertParagraph().Append("TEST");
    title.Alignment = Alignment.center;        
    Paragraph p1 = document.InsertParagraph();
    p1.AppendPicture(picture);                
    document.Save();
}