configuration>
  appSettings>
    add key="dmPic" value="bgImages/moren.jpg" />
  /appSettings>
/configuration>

if (sdr.Read())
{
    this.lblDianMianMingCheng.Text = sdr["BuJuGongSiName"].ToString();
    DianMianId = sdr["BuJuID"].ToString();
    if (sdr["BuJuCarPhoto"].ToString() == "")
    {
        this.ImageDianMian.ImageUrl = ConfigurationManager.AppSettings["dmPic"].ToString();//重点之处
    }
    else {

        this.ImageDianMian.ImageUrl = sdr["BuJuCarPhoto"].ToString();
    }            
}