XML Serialization Windows Phone 8

I'm trying to serialize my ObservableCollection in XML file.

"Memos" class is this:

[DataContract]
public class Memos
{
    [DataMember]
    public string Date { get; set; }

    [DataMember]
    public string Event { get; set; }

    public BitmapImage Photo { get; set; }

    [DataMember]
    public string Path { get; set; }
    public Memos() { }

    public static async Task<BitmapImage> LoadImage(StorageFile file)
    {
        //code
    }
}

If I leave "Photo" field, it works properly. But I don't understand why, because I don't mark it with [DataMember] property.

This is my code

using (var file = storage.CreateFile("Memos.xml"))
{
    var serializer = new XmlSerializer(typeof(ObservableCollection<Memos>));
    serializer.Serialize(file, MainPage.ListMemos);
}