Page 5:1st poster -
Kamikaze Kendra:
Page 10: 3rd poster
Amane:
Page 15: 7th poster
*Hime*:
Page 20: 2nd poster Hime
Page 25: 15th poster Amane
Page 30: 5th poster
evil_samara:
Page 35: 20th poster Hime
Page 40: 11th poster
Divacita:
Page 45: 14th poster @superzombiepotato;
Page 50: 4th poster: Kay
Page 50: Full art for poster 5!:Superzombiepotato
Page 55: 22nd poster Pistachio Mustache
Page 60: 25th poster
Every 5 pages there is the chance of getting a free art that will look along these lines.
Bribery for a slot can also be had if you don't want to wait to see if you win.