I like it - but what's going on with her foot? There's a weird shadow on it, or maybe it's just turned against the light at an odd angle. I'm viewing it on my iPhone though so it may not be a big deal on a different screen.
Tree in the background looks pretty much vertical to me, I think it's pretty much square. Since there's a lack of obvious cues, getting it exact shouldn't matter much.
I like this image, but I feel that the massive differences in tonality may hold it back as a good B&W. Perhaps if the subject were more isolated from the background it would work better as a B&W. I would be interested to see a color version for comparison.