How do I can display images with the code behind C# ?

You can use this code to bind image(s) to your Content place holder. If you want more than one image then put it in a loop.
You can assign Image URL by many ways. I have shown some ways. Hope it is useful to you.

protected void btnImportImages_Click(object sender, EventArgs e)
        {
            Image im = new Image();
            im.ID = "image";
            im.ImageUrl = "~/Image.JPG/";

            //alternatives for giving path ☺
            //im.ImageUrl=(@"C:\Image.JPG");
            //im.ImageUrl = Server.MapPath("~/Imagepath/ImageName.jpg");
            PlaceHolder1.Controls.Add(im);

        }