I guess I went with Nausicaa (from the Studio Ghibli film "Nausicaa of the Valley of the Wind,") because I relate to her. Like me, she's a lover of nature, a very calm soul, pacifist, always tries to see the best in people, and I love those scenes in the movie where she's just out in the forest, walking around, and taking deep breaths as she appreciates the wonders around her. I totally do that all the time! (Plus I wish I could fly like her, but oh well...)
If you mean why I chose her over a real picture of myself, though, it's just because I'm VERY early in HRT, and still really uncomfortable about my "girl mode," so I'd just rather have a picture that is completely female already rather than me in some awkward between-genders state. This may change once HRT has worked its magic, but for now I just feel better going with Nausicaa. It feels like she's actually a better representation of my true female self than reality is.