试试这个：当用户单击Map时，捕获光标的位置（在“screen”坐标中）以及Map控件的当前位置。现在，当您移动鼠标（且仅当LButton按下时）计算当前光标位置的增量（在“屏幕”坐标系中）相对于光标位置捕捉到的MouseDown事件.然后，将Map控件的新位置设置为在MouseDown事件中捕获的位置加上delta的总和。
使其正常工作的一个关键是获取处理程序事件的e.Location，并将该点转换为 * 相对于发送者 * 的屏幕坐标。

public MainForm()
{
    InitializeComponent();
    countryMapImage.MouseMove += onMapMouseMove;
    countryMapImage.MouseDown += onMapMouseDown;
}

Point
    // Where's the cursor in relation to screen when mouse button is pressed?
    _mouseDownScreen = new Point(),
    // Where's the 'map' control when mouse button is pressed?
    _controlDownPoint = new Point(),
    // How much has the mouse moved from it's original mouse-down location?
    _mouseDelta = new Point();

private void onMapMouseDown(object sender, MouseEventArgs e)
{
    if (sender is Control control)
    {                
        _mouseDownScreen = control.PointToScreen(e.Location);
        Text = $"{_mouseDownScreen}";
        _controlDownPoint = countryMapImage.Location;
    }
}

private void onMapMouseMove(object sender, MouseEventArgs e)
{
    if (MouseButtons.Equals(MouseButtons.Left))
    {
        if (sender is Control control)
        {
            var screen = control.PointToScreen(e.Location);
            _mouseDelta = new Point(screen.X - _mouseDownScreen.X, screen.Y - _mouseDownScreen.Y);
            Text = $"{_mouseDownScreen} {screen} {_mouseDelta}";
            var newControlLocation = new Point(_controlDownPoint.X + _mouseDelta.X, _controlDownPoint.Y + _mouseDelta.Y);
            if(!countryMapImage.Location.Equals(newControlLocation))
            {
                countryMapImage.Location = newControlLocation;
            }
        }
    }
}

在这个简单的场景中，不需要处理MouseUp事件。