I think the only problem of this wallpaper is that the vector looks too jaggy in a few parts, and the fact the outlines are probably too thick for that kind of background, which is why there seems to be an unpleasing contrast between the vector and the background. But colour-wise, lightning-wise, it's pretty good and I don't mind having the background that simple, it's a desktop wallpaper afterall, and Yoko is the focal point of the composition anyway. But it's still a nice job anyway.

And you might want to credit the original artists who did that artwork of Yoko, because that is not official art by any of Gainax's animators/artists (all their official artworks are HERE), but it's rather a doujin/fanart of Yoko (well done, but still fanart).