NHibernate Left Outer Join SubClass

I have 2 product and image objects. Not all images are product images, and images are a subclass of the file below, these are my entities. I need to find all images that are not related to the product. I am new to object lookup and have tried many approaches. Any ideas or links would be greatly appreciated.

public class File
    {
        #region Feilds
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
        public virtual Enumerations.File.FileType Type { get; set; }
        public virtual string Extension { get; set; }
        public virtual string Path { get; set; }
        public virtual DateTime DateCreated { get; set; }
        public virtual DateTime DateModified { get; set; }
        #endregion
    }

public class Image : File
{
    #region Fields
    public virtual string ImageName { get; set; }
    public virtual string Description { get; set; }
    public virtual bool Active { get; set; }
    public virtual DateTime DateTaken { get; set; }
    #endregion
}

public class Product
{
    #region Properties
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual string Description { get; set; }
    public virtual decimal Price { get; set; }
    public virtual decimal Weight { get; set; }
    public virtual bool IsDigital { get; set; }
    public virtual DateTime DateCreated { get; set; }
    public virtual IList<Category> ProductCategories { get; set; }
    public virtual IList<ProductAttribute> ProductAttributes { get; set; }
    public virtual IList<Image> ProductImages { get; set; }
    #endregion
}