There is an artbook that features official +18 art from High School DxD, Ikkitousen, and School Days. However, There’s no trace of it online; the only way to get it is by buying it. The name of the artbook is "ごとうじゅんじのアイキャッチすぺしゃる【ハイスクールDxD 、 一騎当千 、 School days】"