How to load and image from Resources into a byte-array?

Hi all
I have a (hopefully ) simple question (I don’t have found an answer, that fit’s by my searches).

I work with Xamarin.Forms 1.4.1-Pre-1.
In my app, I have:

byte[] AvatarErfassung; // Bytearray, to later update the webservice
var Avatar = new Image { HeightRequest = 71, WidthRequest = 61, HorizontalOptions = LayoutOptions.Start };
Avatar.Source = "SymbolMann.jpg";  

where the image "SymbolMann.jpg” is included as project-ressource and showed on a page (without problems).
I now want to put the image in a byte-array to send it to our webservice.

Question:
How to get the image “SymbolMann.jpg” into the byte-array “AvatarErfassung” ?

Thanks for every answer.