There are a lot of ways to store a picture. If the property will be binding to an Image Control. Then you can use: byte[], BitmapImage, string(Uri) to store a picture. This is because those types can automatic converter a BitmapImage to ImageSource.
In this case, you just need to modify the ImageSource To BitmapImage.
By the way, The reason why you can't use ImageSource is the Constructor of ImageSource is internal, so you can't even create an ImageSource object.
So, that's all, try to use some other type.
public class Card
{
// Try some other type, they all can be bind to Image.Source.
public BitmapImage image;
}
Hope it helpful.