Image swapping in Panel

private void panel_MouseDown(object sender, MouseEventArgs e)
{
    Panel source = (Panel) sender;
    DoDragDrop(source.BackgroundImage, DragDropEffects.Copy);
    source.BackgroundImage = b;
}
private void panel_DragEnter(object sender, DragEventArgs e)
{
    if (e.Data.GetDataPresent(typeof(Bitmap)))
    {
        p = (Panel) sender;
        e.Effect = DragDropEffects.Copy;
    }
    else
    {
        e.Effect = DragDropEffects.None;
    }
}
private void panel_DragDrop(object sender, DragEventArgs e)
{
    Panel destination = (Panel) sender;
    b = (Bitmap) destination.BackgroundImage;
    destination.BackgroundImage = (Bitmap) e.Data.GetData(typeof(Bitmap));
}