Use a parameterized statement and many problems will go away.
But if you think you are storing an image in the database, you will have to do more than pass the name of the file.
SqlCommand cmd = new SqlCommand("insert into tblStudent(std_ID,std_Name,std_Password,std_ContactNumber,std_Address,std_Image,module_ID,intake_Code) Values(@std_ID,@std_Name,@std_Password,@std_ContactNumber,@std_Address,@std_Image,@module_ID,@intake_Code)", cn);
cmd.Parameters.AddWithVlaue ( "@std_ID" , TextBox2.Text ) ;
cmd.ExecuteNonQuery();