private int   currentX,currentY;
private bool  isDragging = false;

private void myPictureBox_MouseDown(object sender,MouseEventArgs e) 
{
  isDragging = true;

  currentX = e.X;
  currentY = e.Y;
}

private void myPictureBox_MouseMove(object sender,MouseEventArgs e) 
{
  if (isDragging) 
  {
    myPictureBox.Top = myPictureBox.Top + (e.Y - currentY);
    myPictureBox.Left = myPictureBox.Left + (e.X - currentX);
  }
}

private void myPictureBox_MouseUp(object sender,MouseEventArgs e) 
{
  isDragging = false;
}