Fastest way to convert Image to Byte array

So is there any other method to achieve this goal?

No. In order to convert an image to a byte array you have to specify an image format - just as you have to specify an encoding when you convert text to a byte array.

If you're worried about compression artefacts, pick a lossless format. If you're worried about CPU resources, pick a format which doesn't bother compressing - just raw ARGB pixels, for example. But of course that will lead to a larger byte array.

Note that if you pick a format which does include compression, there's no point in then compressing the byte array afterwards - it's almost certain to have no beneficial effect.

There is a RawFormat property of Image parameter which returns the file format of the image. You might try the following:

// extension method
public static byte[] imageToByteArray(this System.Drawing.Image image)
{
    using(var ms = new MemoryStream())
    {
        image.Save(ms, image.RawFormat);
        return ms.ToArray();
    }
}