How do i make zoom in/out for a drawn rectangle over a pictureBox1 ?

The idea is that im drawing a rectangle over the pictureBox1 and then when i move the mouse wheel up/down it will resize like zoom in/out the area of the image in the pictureBox1 that i drawed the rectangle. 

So far i did that i can draw a rectangle. And i did that i can zoom in/out the image in the pictureBox1 but i couldn't find how to make that it will zoom in/out only the part of the rectangle and not the whole image in the pictureBox1.

This is the code.

At the top of the form i did:

double increment = 1.25;
Image img;
Rectangle mRect;
private double factor = 1;
private double currentfactor;

Then in the constructor i did:

pictureBox1.MouseWheel += new MouseEventHandler(pictureBox1_MouseWheel);
pictureBox1.MouseHover += new EventHandler(pictureBox1_MouseHover);
this.DoubleBuffered = true;
bitmapwithclouds = new Bitmap(Form1PictureBox1Image);
currentfactor = factor;
pictureBox1.Image = ResizeImage(bitmapwithclouds, new Size((int)(bitmapwithclouds.Width * factor), (int)(bitmapwithclouds.Height * factor)));
img = new Bitmap(pictureBox1.Image);

Then the ResizeImage method:

private Image ResizeImage(Image img, Size size)
        {
            return new Bitmap(img, size);
        }

pictureBox1 paint event:

private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            DrawRectangle(e.Graphics);
        }

DrawRectangle method:

private void DrawRectangle(Graphics e)
        {
            using (Pen pen = new Pen(Color.Red, 2))
            {
                e.DrawRectangle(pen, mRect);
            }
        }

pictureBox1 mouse down event:

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            mRect = new Rectangle(e.X, e.Y, 0, 0);
            pictureBox1.Invalidate();
        }

pictureBox1 mouse move event:

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                mRect = new Rectangle(mRect.Left, mRect.Top, e.X - mRect.Left, e.Y - mRect.Top);
                pictureBox1.Invalidate();
            }
        }

picutreBox1 mousewheel event:

private void pictureBox1_MouseWheel(object sender, MouseEventArgs e)
        {
            Image im = new Bitmap(mRect.Width, mRect.Height);
            CalculateNewSizeFactor(e.Delta);
            if (pictureBox1.Image != null) pictureBox1.Image.Dispose();
            pictureBox1.Image = null;
            pictureBox1.Image = ResizeImage(im, new Size((int)(im.Width * currentfactor), (int)(im.Height * currentfactor)));
        }

pictureBox1 mouse hover event:

void pictureBox1_MouseHover(object sender, EventArgs e)
        {
            pictureBox1.Focus();
        }

And the method calculate new size factor:

private void CalculateNewSizeFactor(int delta)
        {
            if (delta > 0 && factor < 2.5)
            {
                factor *= increment;
                currentfactor = factor;
            }
            else if (delta < 0 && factor > 0.25)
            {
                factor /= increment;
                currentfactor = factor;
            }
        }

Now what i changed few mintues ago was in the mouse wheel event:

I added this line:

Image im = new Bitmap(mRect.Width, mRect.Height);

And replaced it with the variable img in this line:

pictureBox1.Image = ResizeImage(im, new Size((int)(im.Width * currentfactor), (int)(im.Height * currentfactor)));

And its not working.

But if i will put instead the variable (im) the variable img i will be able to zoom in out the whole image in the pictureBox.

But since i want to zoom in out only the part of the drawed rectangle i tried to make this changes but it dosent work at all. Show me whie and dosent zoom in out at all.