First, I must say that I like this drawing. The theme, though popular, is not generic - Especially this reference picture. Your technique is also very good. I like the use of dark shades contrasting the lighter ones, which adds to the vision. My favourite part is Harry's face which in my opinion is the best drawn - Especially the spectacles. Also, I like the details in the sword, and the lack of details in the background: It helps concentrating on the characters.
I think, however, that in some places you used too sticking out black lines: It can be mostly seen in Ron's collar, but also in some other parts. Also, Hermione's hand looks less realistic than the rest of the drawing, probably in part because of the dark contour.
Brilliant drawing! You should definitely get a better photo or scan of it.
I really don't see anything that can be improved here. You captured very well the affect of the fur, as well as that of sparkling parts - such as the noses and the eye of the left wolf. Actually, the noses are my favourite part <img src="e.deviantart.com/emoticons/s/s…" width="15" height="15" alt="" title=" (Smile)" /> I also like the way you chose to place their heads with regards to the paper, the look from a close distance adds a lot to the impact. Also, the black background contrast well with the comparatively light faces of theirs.
It's hard to write a 100 words long critique about a drawing you see nothing that can be improved about, but somehow I succeeded <img src="e.deviantart.com/emoticons/w/w…" width="15" height="15" alt="" title=" (Wink)" />