From the virtually unknown manga Et Cetera which my friend recently introduced me to.

http://en.wikipedia.org/wiki/Et_Cetera

It's actually a very cute story with some interesting characters.
The old west theme is reminiscent of Trigun (though I don't know which came first).

The main character is Mingchao, the girl depicted in the wallpaper.

The scan is from the manga, colored by me.