Check out example codes for "how to convert a Bitmap to a base64 string c# xamarin universal". It will help you in understanding the concepts better.

Code Example 1

using(var stream = new MemoryStream())
{
   yourBitmap.Compress(Bitmap.CompressFormat.Png,0, stream);

    var bytes = stream.ToArray();
    var str = Convert.ToBase64String(bytes);
}

Code Example 2

Bitmap bitmap = BitmapFactory.DecodeResource(Resources, Resource.Drawable.ace1);
MemoryStream stream = new MemoryStream();
bitmap.Compress(Bitmap.CompressFormat.Jpeg, 100, stream);
byte[] ba = stream.ToArray();
string bal = Base64.EncodeToString(ba, Base64.Default);

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