I assume this is just a copy/paste typo
namespace Nop.Data.Mapping.Catalog
{
public partial class ProductPictureMap : EntityTypeConfiguration<ProductPicture>
{
public ProductPictureMap()
{
this.ToTable("Product_Picture_Mapping");
this.HasKey(pp => pp.Id);
this.HasRequired(pp => pp.Picture)
.WithMany(p => p.ProductPictures)
.HasForeignKey(pp => pp.PictureId);
this.HasRequired(pp => pp.Product)
.WithMany(p => p.ProductPictures)
.HasForeignKey(pp => pp.ProductId);
}
}
}