moving figure in a range

I have a visual c# program to move channela bmp in y axis , it can moved upwards and downwards in a limit value , moving up to a limit value can be done while downwards cannot , can someone help me ?

        private void channela_MouseDown(object sender, MouseEventArgs e)
        {
            drag = true;
            y_down = e.Y;
        }

        private void Channela_MouseMove(object sender, MouseEventArgs e)
        {

            if (drag)
            {
                int topPosition = channela.Top + e.Y - y_down;
                   channela.Top = Math.Max(80, topPosition);
                       if (channela.Bottom == 477
                        {
                         channela.Top = Math.Max(477,topPosition);
                         return;
                          }
                           }

        }

Assuming channela is a picturebox
public partial class Form1 : Form
{
bool drag = false;
Point pt;
public Form1()
{
InitializeComponent();
}
private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
drag = true;
pt = e.Location;
}
private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
{
if (drag == true)
{
Control c = (Control)sender;
c.Left = (c.Left + e.X) - pt.X;
c.Top = (c.Top + e.Y) - pt.Y;
c.BringToFront();
if (c.Top < 0)
{
c.Top = 0;
drag = false;
}
else if (c.Top > (this.ClientSize.Height - c.Height))
{
c.Top = (c.Top - 1);
drag = false;
}
if (c.Left < 0)
{
c.Left = 0;
drag = false;
}
else if (c.Left > (this.ClientSize.Width - c.Width))
{
c.Left = (c.Left - 1);
drag = false;
}
}
}
private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
{
drag = false;
}
}