Word.Application app = new Word.Application();
var doc = app.Documents.Open(@"C:\Users\SomeUserName\Desktop\Doc1.docx");

var shape = doc.Bookmarks["PicHere"].Range.InlineShapes.AddPicture(@"C:\Users\SomePicture\Pictures\1234.JPG",false,true);
shape.Width = 150;
shape.Height = 150;
app.Visible = true;