GDI+画图问题

我是用变量画的，画在了windows窗口里面。主要代码如下：
 private void Frm_Main_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)//如果按下鼠标左键
            {
                //如果选择文字输入，则打开strInput窗体
                if (drawTool == drawTools.String)
                {
                    Frm_Text inputBox = new Frm_Text();
                    inputBox.StartPosition = FormStartPosition.CenterParent;
                    if (inputBox.ShowDialog() == DialogResult.OK)
                    {
                        Graphics g = this.CreateGraphics();
                        Font theFont = this.Font;
                        g.DrawString(inputBox.textBox1.Text, theFont, new SolidBrush(foreColor), e.X, e.Y);
                        ig.DrawString(inputBox.textBox1.Text, theFont, new SolidBrush(foreColor), e.X, e.Y);
                    }
                }
                //如果开始绘制，则开始记录鼠标位置
                else if ((isDrawing = !isDrawing) == true)
                {
                    startPoint = new Point(e.X, e.Y);
                    oldPoint = new Point(e.X, e.Y);
                }
            }
        }
 private void Frm_Main_MouseMove(object sender, MouseEventArgs e)
        {
            Graphics g;
            g = this.CreateGraphics();

            if (isDrawing)//如果开始绘图
            {
                switch (drawTool)
                {
                    case drawTools.None:
                        break;
                    case drawTools.Pen:
                        //从上一个点到当前点绘制线段
                        g.DrawLine(new Pen(foreColor, 1), oldPoint, new Point(e.X, e.Y));
                        ig.DrawLine(new Pen(foreColor, 1), oldPoint, new Point(e.X, e.Y));
                        oldPoint.X = e.X;
                        oldPoint.Y = e.Y;
                        break;
                    case drawTools.Line:
                        //首先恢复此次操作之前的图像，然后再添加Line
                        this.Frm_Main_Paint(this, new PaintEventArgs(this.CreateGraphics(), this.ClientRectangle));
                        g.DrawLine(new Pen(foreColor, 1), startPoint, new Point(e.X, e.Y));
                        oldPoint.X = e.X;
                        oldPoint.Y = e.Y;
                        break;
                    case drawTools.Ellipse:
                        //首先恢复此次操作之前的图像，然后再添加Ellipse
                        this.Frm_Main_Paint(this, new PaintEventArgs(this.CreateGraphics(), this.ClientRectangle));
                        g.DrawEllipse(new Pen(foreColor, 1), startPoint.X, startPoint.Y, e.X - startPoint.X, e.Y - startPoint.Y);
                        oldPoint.X = e.X;
                        oldPoint.Y = e.Y;
                        break;
                    case drawTools.Rectangle:
                        //首先恢复此次操作之前的图像，然后再添加Rectangle
                        this.Frm_Main_Paint(this, new PaintEventArgs(this.CreateGraphics(), this.ClientRectangle));
                        g.DrawRectangle(new Pen(foreColor, 1), startPoint.X, startPoint.Y, e.X - startPoint.X, e.Y - startPoint.Y);
                        oldPoint.X = e.X;
                        oldPoint.Y = e.Y;
                        break;
                    case drawTools.String:
                        break;
                    case drawTools.Rubber:
                        //用背景色绘制宽线段
                        g.DrawLine(new Pen(backColor, 100), oldPoint, new Point(e.X, e.Y));
                        ig.DrawLine(new Pen(backColor, 100), oldPoint, new Point(e.X, e.Y));
                        oldPoint.X = e.X;
                        oldPoint.Y = e.Y;
                        break;
                    case drawTools.triangle:
                        //首先恢复此次操作之前的图像，然后再添加triangle
                        this.Frm_Main_Paint(this, new PaintEventArgs(this.CreateGraphics(), this.ClientRectangle));
                        Pen mypen = new Pen(foreColor,1);
                        Point point1 = new Point(startPoint.X,e.Y);
                        Point point2 = new Point((startPoint.X+e.X)/2,startPoint.Y);
                        Point point3 = new Point(e.X,e.Y);
                        Point[] mypoints = {point1,point2,point3 };
                        g.DrawPolygon(mypen,mypoints);
                        oldPoint.X = e.X;
                        oldPoint.Y = e.Y;
                        break;
                }
            }
        }
private void Frm_Main_MouseUp(object sender, MouseEventArgs e)
        {
            isDrawing = false;
            switch (drawTool)
            {
                case drawTools.Line://画线
                    ig.DrawLine(new Pen(foreColor, 1), startPoint, new Point(e.X, e.Y));
                    break;
                case drawTools.Ellipse://画椭圆
                    ig.DrawEllipse(new Pen(foreColor, 1), startPoint.X, startPoint.Y, e.X - startPoint.X, e.Y - startPoint.Y);
                    break;
                case drawTools.Rectangle://画矩形
                    ig.DrawRectangle(new Pen(foreColor, 1), startPoint.X, startPoint.Y, e.X - startPoint.X, e.Y - startPoint.Y);
                    break;
                case drawTools.triangle://画三角形
                     Pen mypen = new Pen(foreColor,1);
                     Point point1 = new Point(startPoint.X,e.Y);
                     Point point2 = new Point((startPoint.X+e.X)/2,startPoint.Y);
                     Point point3 = new Point(e.X,e.Y);
                     Point[] mypoints = {point1,point2,point3 };
                     ig.DrawPolygon(mypen,mypoints);
                     break;
            }
        }
应该再写哪些代码，才能选中所画的图，并移动？