While I do not develop Win Store apps, a quick look at the documentation for 'SaveJpeg suggests you are missing arguments: [
^].
SaveJpeg(
this WriteableBitmap bitmap,
Stream targetStream,
int targetWidth,
int targetHeight,
int orientation,
int quality
)
It also "jumps out at me" that you create a memory stream and then do not do anything to get the image data into the stream.
Typical code to get Image contents as byte[] in System.Drawing.Image:
MemoryStream ms = new MemoryStream();
YourImage.Save(ms,System.Drawing.Imaging.ImageFormat.Jpeg);
byte[] bAry = ms.ToArray();