web. config:

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

Corresponding.cs page:

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