From MSDN:
Windows Phone 8: This API is not intended to be used directly from your code.
You can use the PhotoChooserTask to let the user pick a photo. Alternatively, you can use MediaLibrary.Pictures to programmatically access the pictures. For example
MediaLibrary ml = new MediaLibrary();
var picture = ml.Pictures.Where(x => x.Name.Equals("WP_20130607_003.jpg")).FirstOrDefault();
if(picture != null)
{
//get the image stream
var picStream = picture.GetImage();
//...
}