C#

public partial class Form1 : Form
  {
      public OpenFileDialog dialog = new OpenFileDialog();
      System.Data.SqlServerCe.SqlCeConnection con;
      System.Data.SqlServerCe.SqlCeDataAdapter da1;
      DataSet ds1;



      public Form1()
      {
          InitializeComponent();
      }

      private void btn_sel_Click(object sender, EventArgs e)
      {

          //dialog.Filter = "*.sdf";
          dialog.ShowDialog();
          MessageBox.Show(dialog.FileName);
          loadconnection();

      }

      private void btn_close_Click(object sender, EventArgs e)
      {
          Application.Exit();
      }

      public void loadconnection()
      {
          con = new System.Data.SqlServerCe.SqlCeConnection();
          con.ConnectionString = "Data Source=" + dialog.FileName + ""; //opening dialog (selected file)

          con.Open();


          ds1 = new DataSet();
          string sql = "SELECT * From tbl_games";
          da1 = new System.Data.SqlServerCe.SqlCeDataAdapter(sql, con); // Modified this line
          da1.Fill(ds1, "Name");

          con.Close();
      }
  }

if (txtdisc.Text == null)
{
    ..
    Double txtdisc22013 = Convert.ToDouble(txtdisc.Text);

Double txtdisc22013 = Convert.ToDouble(null);

 if(string.IsNullorEmpty(txtdic.Text) != true)
 {
   //...
 }

or 

 if(txtdisc.Text != string.Empty)
 {
  //...
 }

Hi
First, like others mentioned, "if" and "else" are conditional statements, they are not loop.
Second, the textbox will return empty if it has no value assigned to it, so checking for null is redundant.
Third, this is a coding horror!

if (txtdisc.Text ==null)  This check is redundant therefore  the calls below will never be executed.
{
   txttot22013 = txtpr22013 + txtpr22014 + txtpr22015 + txtpr22016 + txtpr22017;

   Double txtdisc22013 = Convert.ToDouble(txtdisc.Text); Converting a null to a nullable Double will 0.0;

   txttot22013 = txttot22013 - txtdisc22013; (Some value – 0.0)

   txttot.Text = txttot22013.ToString();
}
else
{
   txttot22013 = txtpr22013 + txtpr22014 + txtpr22015 + txtpr22016 + txtpr22017;

   txttot.Text = txttot22013.ToString(); The value of "txttot22013" in the else conditions  is no different to the one in if condition.
}

if (txtvat.Text == null)
{
   MessageBox.Show("Enter Vat Amount"); This will never be called.
}
else
{
   Double txtvat22013 = Convert.ToDouble(txtvat.Text); Possible exception.

   txtvat22013 = txtvat22013 / 100;

   txtvat22013 = txtvat22013 * txttot22013;

   Double txtvatinclu22013 = txtvat22013 + txttot22013;

   txtvatinclu.Text = txtvatinclu22013.ToString();
}


If I rewrite your code, it may as follows:

txttot22013 = txtpr22013 + txtpr22014 + txtpr22015 + txtpr22016 + txtpr22017;
txttot.Text = txttot22013.ToString();

if (string.IsNullOrEmpty(txtvat.Text))
{
    MessageBox.Show("Enter Vat Amount");
}
else
{
    double txtvat22013;
    if (Double.TryParse(txtvat.Text, txtvat22013))
    {
        Double txtvatinclu22013 = (txtvat22013 * txttot22013 / 100) + txttot22013;

        txtvatinclu.Text = txtvatinclu22013.ToString();
    }else
    {
    MessageBox.Show("Enter Valid Vat Amount");
    }
}

Bitmap b = new Bitmap(this.imageList1.Images[0]);
            b.Save(@"Drive:\path\image-file-name.bmp");