private static byte[] ConvertPngToJpg(byte[] pngByteArray)
{
    MemoryStream pngStream = new MemoryStream(pngByteArray);
    Image returnImage = Image.FromStream(pngStream);
    MemoryStream jpgStream = new MemoryStream();
    returnImage.Save(jpgStream ,ImageFormat.Jpeg);
    byte[] returnValue = StreamToByteArray(jpgStream);
            
    return returnValue;
}

// This Code is from http://stackoverflow.com/questions/1080442/how-to-convert-an-stream-into-a-byte-in-c
public static byte[] StreamToByteArray(Stream stream)
{
    long originalPosition = stream.Position;
    stream.Position = 0;

    try
    {
        byte[] readBuffer = new byte[4096];

        int totalBytesRead = 0;
        int bytesRead;

        while ((bytesRead = stream.Read(readBuffer, totalBytesRead, readBuffer.Length - totalBytesRead)) > 0)
        {
            totalBytesRead += bytesRead;

            if (totalBytesRead == readBuffer.Length)
            {
                int nextByte = stream.ReadByte();
                if (nextByte != -1)
                {
                    byte[] temp = new byte[readBuffer.Length * 2];
                    Buffer.BlockCopy(readBuffer, 0, temp, 0, readBuffer.Length);
                    Buffer.SetByte(temp, totalBytesRead, (byte)nextByte);
                    readBuffer = temp;
                    totalBytesRead++;
                }
            }
        }

        byte[] buffer = readBuffer;
        if (readBuffer.Length != totalBytesRead)
        {
            buffer = new byte[totalBytesRead];
            Buffer.BlockCopy(readBuffer, 0, buffer, 0, totalBytesRead);
        }
        return buffer;
    }
    finally
    {
        stream.Position = originalPosition;
    }
}

/// <summary>
/// Convert a png byte-array to jpg byte-array
/// </summary>
/// <param name="pngByteArray">png image as byte[]</param>
/// <returns></returns>
public static byte[] ConvertPngToJpg(byte[] pngByteArray)
{
    Image img;
    using (MemoryStream msPng = new MemoryStream(pngByteArray))
    {
        img = Image.FromStream(msPng);
    }

    byte[] jpgArray;
    using (MemoryStream msJpeg = new MemoryStream())
    {
        img.Save(msJpeg, ImageFormat.Jpeg);
        jpgArray = msJpeg.ToArray();
    }

    return jpgArray;
}