Bitmap Image from Hex string - extra bytes being added

I have a Hex string that's coming from postscript file.  
(This Hex string is for image - I need to modify RGB color values for this image and write it back to postscript file)  

Below are the methods that I am using. I have commented out the method for updating color.

    public static void ProcessImageColourMapping()
    {
        string imageDataSource = "803fe0503824160d0784426150b864361d0f8844625138a4562d178c466351b8e4763d1f904864523924964d27944a6552b964b65d2f984c665339a4d66d379c4e6753b9e4f67d3fa05068543a25168d47a4526954ba648202";
        string imageDataSourceUpdated = GetUpdatedImage(imageDataSource);
    }

    public static string GetUpdatedImage(string strImageDataSource)
    {
        string imageDataSourceUpdated = "";

        byte[] imageBytes = StringToByteArray(strImageDataSource);
        Bitmap bitmapImage = ByteArrayToBitmap(imageBytes);
        //UpdateColour(bitmapImage);
        byte[] imageBytesUpdated = BitmapToByteArray(bitmapImage);
        imageDataSourceUpdated = ByteArrayToString(imageBytesUpdated);

        return imageDataSourceUpdated;
    }

    public static byte[] StringToByteArray(String imageHexString)
    {
        int numberOfChars = imageHexString.Length / 2;
        byte[] byteArray = new byte[numberOfChars];
        using (var sr = new StringReader(imageHexString))
        {
            for (int i = 0; i < numberOfChars; i++)
                byteArray[i] = Convert.ToByte(new string(new char[2] { (char)sr.Read(), (char)sr.Read() }), 16);
        }
        return byteArray;
    }

    public static Bitmap ByteArrayToBitmap(byte[] byteArray)
    {
        int width = 986; //width and height are taken from postscript file for testing a single hex string.
        int height = 1;
        Bitmap bitmapImage = new Bitmap(width, height, PixelFormat.Format32bppPArgb);
        BitmapData bmpData = bitmapImage.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.ReadWrite, PixelFormat.Format32bppPArgb);            
        try
        {                
            Marshal.Copy(byteArray, 0, bmpData.Scan0, byteArray.Length);
        }
        finally
        {
            bitmapImage.UnlockBits(bmpData);                
        }
        return bitmapImage;
    }

    public static byte[] BitmapToByteArray(Bitmap bitmap)
    {
        BitmapData bmpdata = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadOnly, bitmap.PixelFormat);
        int numbytes = bmpdata.Stride * bitmap.Height;
        byte[] bytedata = new byte[numbytes];
        try
        {
            Marshal.Copy(bmpdata.Scan0, bytedata, 0, numbytes);            
        }
        finally
        {
            bitmap.UnlockBits(bmpdata);
        }
        return bytedata;
    }

    public static string ByteArrayToString(byte[] byteArray)
    {
        StringBuilder hex = new StringBuilder(byteArray.Length * 2);
        foreach (byte b in byteArray)
        {
            hex.AppendFormat("{0:x2}", b);
        }
        return hex.ToString();
    }

**Issue:**  
In below code, I am not updating anything for incoming Hex string `imageDataSource`.  
Converting it to byte[] - then to Bitmap - Back to byte[] - and finally back to Hex string.

So, `imageDataSourceUpdated` should have same value as `imageDataSource`.   
However, when I finally check the value for `imageDataSourceUpdated`, it comes out as:  

`803fe0503824160d0784426150b864361d0f8844625138a4562d178c466351b8e4763d1f904864523924964d27944a6552b964b65d2f984c665339a4d66d379c4e6753b9e4f67d3fa05068543a25168d47a4526954ba64820200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000`..... 
So many zeros being appended.

Can please guide what I am missing here.