Is there a cross-platform solution to ImageSource to byte[]?

I did researches and fell on this solution: http://forums.xamarin.com/discussion/22682/is-there-a-way-to-turn-an-imagesource-into-a-byte-array

We want to upload an image through a HTTP Post, here's what we tried:

HttpClient httpClient = new HttpClient ();
byte[] TargetImageByte = TargetImageSource; //How to convert it to a byte[]?
HttpContent httpContent = new ByteArrayContent (TargetImageByte);
httpClient.PostAsync ("https://api.magikweb.ca/debug/file.php", httpContent);

We also are having a hard time with the libraries we gotta include in the using clauses. It seems like using System.IO; works, but it doesn't give us access to classes like FileInfo or FileStream.
Anybody got a working example?