skipping images ddl gridview

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ADO.NET

There is only a drop down list with student ID and a gridview with students' images in my web page.
If I select a student GridView has to show all the images of the selected student.
Problem is when I select a student my gridview is skipping first two values.
I mean if there are 6 images gridview shows only 4 images.
Here is my code:

private void BindGrid()
    {
        MySqlConnection con = new MySqlConnection(constr);
        MySqlCommand cmd = new MySqlCommand("SELECT * FROM images where Image_ID in (" + String.Join(",", getImage_ID()) + ")", con);
        MySqlDataAdapter da = new MySqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        da.Fill(dt);
        gvImages.DataSource = dt;
        gvImages.DataBind();
    }
    private List<int> getImage_ID()
    {
        List<int> i = new List<int>();
        MySqlConnection con = new MySqlConnection(constr);
        con.Open();
        string query = "Select Come_Image_ID1, Leave_Image_ID from register where Students_ID='" + getStudents_ID() + "' AND Come_Image_ID IS NOT NULL AND Leave_Image_ID IS NOT NULL"; 
        MySqlCommand cmd = new MySqlCommand(query);
        cmd.Connection = con;
        MySqlDataReader reader = cmd.ExecuteReader();
        while (reader.Read())
        {
            foreach (DbDataRecord s in reader)
            {
                i.Add(s.GetInt32(0));
                i.Add(s.GetInt32(1));
            }
        }
        reader.Close();
        return i;
    }

What is the best coding practice for this issue?
Examples would be nice.