They're very good. The first one, however, it seems there's something wrong with the picture of Yuna on the right - it's too wide. And the third one, Tifa's middle picture looks too thin for its height.

Just an idea for the last sig. How about making it so that the two lines on the foreground were behind Rikku so they wouldn't show on her head? I know it's just a minor detail... but yeah. I also like how the S is different than the rest of the name on the last sig. Great job!