The following is how I solved the issue:
byte[] bytearray = null;
using (var ms = new MemoryStream())
{
if (imgPhoto.Source != null)
{
var wbitmp = new WriteableBitmap((BitmapImage) imgPhoto.Source);
wbitmp.SaveJpeg(ms, 46, 38, 0, 100);
bytearray = ms.ToArray();
}
}
if (bytearray != null)
{
Sighting.Instance.ImageData = Convert.ToBase64String(bytearray);
PhotoModel.Instance.BitmapImage = bitmapImage;
}