Despite being shy and facing away from us, the character in the picture is obviously Yuffie. If you're still not convinced, then the accompanying writing definitely includes her name. It's official, she's in the game. As for Shizune, we're intrigued to find out why she's such a prominent figure in the FFVII prequels, but never shows up in Final Fantasy VII itself. Presumably we'll find out when we finally get to play Crisis Core.
[UPDATE] Shizune information corrected. She first appeared in Final Fantasy VII: Before Crisis, not in Advent Children. Sorry for the error.