Program.cs

using System;
using System.IO;
using System.IO.Compression;
using System.Text;
using System.Windows.Media.Imaging;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            const string imageFlePath = @"C:\Users\Alberto\SkyDrive\TVShow-backup.jpg";
            var comments = "{nome:'Alberto'}";

            string jpegDirectory = Path.GetDirectoryName(imageFlePath);
            string jpegFileName = Path.GetFileNameWithoutExtension(imageFlePath);

            BitmapDecoder decoder = null;
            BitmapFrame bitmapFrame = null;
            BitmapMetadata metadata = null;
            FileInfo originalImage = new FileInfo(imageFlePath);

            if (File.Exists(imageFlePath))
            {
                // load the jpg file with a JpegBitmapDecoder    
                using (Stream jpegStreamIn = File.Open(imageFlePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
                {
                    decoder = new JpegBitmapDecoder(jpegStreamIn, BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.OnLoad);
                }

                bitmapFrame = decoder.Frames[0];
                metadata = (BitmapMetadata)bitmapFrame.Metadata;

                if (bitmapFrame != null)
                {
                    BitmapMetadata metaData = (BitmapMetadata)bitmapFrame.Metadata.Clone();

                    if (metaData != null)
                    {
                        // modify the metadata   
                        //metaData.SetQuery("/app1/ifd/exif:{uint=40092}", comments);
                        var query = metaData.GetQuery("/app1/ifd/exif:{uint=40092}");

                        // get an encoder to create a new jpg file with the new metadata.      
                        JpegBitmapEncoder encoder = new JpegBitmapEncoder();
                        encoder.Frames.Add(BitmapFrame.Create(bitmapFrame, bitmapFrame.Thumbnail, metaData, bitmapFrame.ColorContexts));
                        //string jpegNewFileName = Path.Combine(jpegDirectory, "JpegTemp.jpg");

                        // Delete the original
                        originalImage.Delete();

                        // Save the new image 
                        using (Stream jpegStreamOut = File.Open(imageFlePath, FileMode.CreateNew, FileAccess.ReadWrite))
                        {
                            encoder.Save(jpegStreamOut);
                        }
                    }
                }
            }
            /*var str = new StreamReader(@"C:\Users\Alberto\Documents\Visual Studio 2012\Projects\ConsoleApplication2\ConsoleApplication2\TextFile1.txt").ReadToEnd();
            var compress = Compress(str);
            Console.WriteLine(compress.Length);
            Console.WriteLine(str.Length);*/
        }

        public static String Compress(String decompressed)
        {
            byte[] data = Encoding.UTF8.GetBytes(decompressed);
            using (var input = new MemoryStream(data))
            using (var output = new MemoryStream())
            {
                using (var gzip = new GZipStream(output, CompressionLevel.Optimal))
                {
                    input.CopyTo(gzip);
                }
                return Convert.ToBase64String(output.ToArray());
            }
        }

        public static String Decompress(String compressed)
        {
            byte[] data = Convert.FromBase64String(compressed);
            using (MemoryStream input = new MemoryStream(data))
            using (GZipStream gzip = new GZipStream(input, CompressionMode.Decompress))
            using (MemoryStream output = new MemoryStream())
            {
                gzip.CopyTo(output);
                StringBuilder sb = new StringBuilder();
                return Encoding.UTF8.GetString(output.ToArray());

            }
        }
    }
}