I have made a page with pics of each character and her seiyuu.
I've indicated Roses and their soeurs, which is something I myself have trouble remembering. I have also listed some other roles each seiyuu has played.
This is what part of the page looks like. Click on the image to see the whole thing.
And here is a group shot of the main cast (click image to enlarge a bit):
front (l-r): Ikezawa Haruna (Yoshino), Ueda Kana (Yumi), Itou Miki (Sachiko), Noto Mamiko (Shimako). back row: Itou Shizuka (Rei), Satou Rina (Tsutako), Saitou Chiwa (Mami), Shimizu Kaori (Noriko), Toyoguchi Megumi (Sei).
The two gaps in the table are characters whose images I haven't found yet -- Hidemi and Miyuki. If you know they have appeared in the show already, please let me know where. And any suggestions for improving the layout or info are welcome.