Setting String as Image Source in C#
You need to specify an ImageSource instead of a string when doing it from code:
Uri uri = new Uri("...", UriKind.Absolute);
ImageSource imgSource = new BitmapImage(uri);
myImage.Source = imgSource;
Unless I've not found the right part of Scott's post that you're looking at, he is binding image source to a url.
Specificaly,
ImageSource = tweet.Element("user").Element("profile_image_url").Value
Which would be something like
http://a0.twimg.com/profile_images/1158022545/mickn_normal.jpeg