Why so many? And isn't there a thread for this already?
http://forums.animesuki.com/showthread.php?t=11904
12. Dog Days
13. Umisho
14. Fanart including many characters from different places.
15. Fanart from Ore no Imōto
16. Arakawa Under the Bridge
17. Not sure...Strike Witches?
18. No idea...probablt Bleach
19. Cowboy Bebop
20. Not sure. not enough to go on.
21. IS <Infinite Stratos>
22. Puella Magi Madoka ☆ Magica
23. I dont know
24. Senki Zesshou Symphogear (just like it says)
EDIT:
14. I can only identify two of the characters. Lower left is Fate Testarossa from Nanoha series. Lower right is Patchouli Knowledge from Touhou (game).