Translate/Detect Rectangle portion of Image from R

I have a Large size image.Since it takes long to process high res images i resize it keeping the aspect ratio.From the resized image i detect a rectangle and i have the coordinates of the rectangle.

 Bitmap ResizekeepAspectRatio(Bitmap imgPhoto, int Width, int Height)
        {
            int sourceWidth = imgPhoto.Width;
            int sourceHeight = imgPhoto.Height;
            int sourceX = 0;
            int sourceY = 0;
            int destX = 0;
            int destY = 0;

            float nPercent = 0;
            float nPercentW = 0;
            float nPercentH = 0;

            nPercentW = ((float)Width / (float)sourceWidth);
            nPercentH = ((float)Height / (float)sourceHeight);
            if (nPercentH < nPercentW)
            {
                nPercent = nPercentH;
                destX = System.Convert.ToInt16((Width -
                              (sourceWidth * nPercent)) / 2);
            }
            else
            {
                nPercent = nPercentW;
                destY = System.Convert.ToInt16((Height -
                              (sourceHeight * nPercent)) / 2);
            }

            int destWidth = (int)(sourceWidth * nPercent);
            int destHeight = (int)(sourceHeight * nPercent);

            Bitmap bmPhoto = new Bitmap(Width, Height,
                              PixelFormat.Format24bppRgb);
            bmPhoto.SetResolution(imgPhoto.HorizontalResolution,
                             imgPhoto.VerticalResolution);

            Graphics grPhoto = Graphics.FromImage(bmPhoto);
            grPhoto.Clear(Color.Red);
            grPhoto.InterpolationMode =
                    InterpolationMode.HighQualityBicubic;

            grPhoto.DrawImage(imgPhoto,
                new Rectangle(destX, destY, destWidth, destHeight),
                new Rectangle(sourceX, sourceY, sourceWidth, sourceHeight),
                GraphicsUnit.Pixel);

            grPhoto.Dispose();
            return bmPhoto;
        }

Is there a way by which i can translate/Map this rectangle to the Large image so that i get the same area.Im doing this to save time.

Some clarification: I have a Large Original Image.. i resize it keeping the aspect ratio and using some processing i get a rectangle portion in it( Just the coordinates).Since the Image quality of this portion is not good i need to find a way to map this coordinate to the large image.

Ok, so if I understand it clear here it is:

You have a viewport in what you select a rectangle and you want to scale this rectangle to the unscaled image.

So we will have a function like this:

public RectangleF TranslateScale(RectangleF CropRectangle, Bitmap imgPhoto)

First of all what we need is to calculate the multiplier to fit the image on the viewport, exactly like your function:

        int sourceWidth = imgPhoto.Width;
        int sourceHeight = imgPhoto.Height;
        int sourceX = 0;
        int sourceY = 0;
        int destX = 0;
        int destY = 0;

        float nPercent = 0;
        float nPercentW = 0;
        float nPercentH = 0;

        nPercentW = ((float)Width / (float)sourceWidth);
        nPercentH = ((float)Height / (float)sourceHeight);
        if (nPercentH < nPercentW)
            nPercent = nPercentH;
        else
            nPercent = nPercentW;

Now that we know the scale percentage we just take the inverse of your function, instead of multiply just divide the rectangle size and position:

        CropRectangle.X /= nPercent;
        CropRectangle.Y /= nPercent;
        CropRectangle.Width /= nPercent;
        CropRectangle.Height /= nPercent

        return CropRectangle;

And that's it, now you have the rectangle scaled to the original image size, you now can crop that rectangle.