public class Car
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ID { get; set; }

    [Required]
    public string Title { get; set; }
}

public class CarImage
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ID { get; set; }

    [Required]
    public int CarID { get; set; }

    [ForeignKey("CarID")]
    public virtual Car Car { get; set; }

    public string FileName { get; set; }

    public bool IsPrimaryImage { get; set; }
}

public class CarImage
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ID { get; set; }

    [Required]
    public int CarID { get; set; }

    [ForeignKey("CarID")]
    public virtual Car Car { get; set; }

    public string FileName { get; set; }
}

public class Car
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ID { get; set; }

    [Required]
    public string Title { get; set; }

    public int? PrimaryImageID { get; set; }

    public virtual CarImage PrimaryImage { get; set; }
}

using (var db = new DataContext())
{
    var car = db.Car.First(x => x.ID == CarID);

    var image = new CarImage
    {
        CarID = car.ID,
        //Car = car,
        IsDeleted = false,
    };

    db.CarImages.Add(image);

    db.SaveChanges();

    if (isFirstImage)
    {
        car.PrimaryImage = image;
        car.PrimaryImageID = image.ID;
    }

    db.SaveChanges();
}

modelBuilder.Entity<CarImages>()
    .HasRequired(x => x.Car)
    .WithOptional(x => x.PrimaryImage)
    .Map(x => x.MapKey("PrimaryImageID"));