The ones on the first page are seasonal bonuses. Play during March to get 2 CGs of Easter Fuck-Bunny Helen. (requires Helen and Amber at lv100+) Play during October to get 3 CGs of Sex Witch Helen. (requires Helen, Kathrine and Heidal at lv100+) Play during December to get 1 CG of Santa's Little Fuck-bucket Helen. (requires lv100+ Helen)
(Nicknames are just for fun.)
The 5 that are still empty after obtaining Delia are reserved for a Countryside girl who has not been implemented yet AFAIK, despite her pictures being included in the files.
Edit: After resetting for Pearls, I was able to discover Leanne in the City, not the Countryside. She is the one beneath Delia the Nun.