This is a beautiful work masterly done.
It calls for the night, the dreams and the magic universes that are inherit to them.

Many have written about the knee and lighting but I defend that these are perfect. Under a certain angle the knee can actually be hidden and the light source, although extremely smooth can visibly be traced to the left top corner of the image.
Furthermore if we compare with the original photo we can see that the left knee doesn't have definition as well and because you based this creation in that model is totally fair that you remain honest to it. In this basis I find some critiques of this work a bit unfair.

The face is beautifully made with the correct eye line towards the butterfly.

Impressively the first thing we look at is not the lady but the actually butterfly that stands so sharply in the middle of a big empty area and then we are guided to the face looking at it. To my opinion this introduction to the piece is so gentle that truly depicts the word that best describes it: gentleness.

Everything is gentle in this piece, from the colours to the shapes.
Marvelous work!

This is just so wonderful to look at but...

The butterfly draws you in yet I'm not sure where the light source is coming from. I'd make the butterfly more illuminous.

The knees are just bugging me. Not sure where the left one is, it gets lost in the folds of fabric.

Her hair at the bottom, you can see the paint strokes and it blurs a little, very minute but it bugged me.

You did the water so beautifully but again, not sure where the light source coming from. It goes light then dark on her dress.

Awesome concept!