Heh, well, so many appropriately enthusiastic comments have already been made. What's one more? I AGREE!
But one of the first things that struck me looking this over is the awesome layout. Your composition! It's full, but not busy. It balances their frantic action (Harry all OMG I TAKE U! and Snape equally OMG but paralyzed - does he shrink away and maybe fall?! EEE! I LOVE TRICK-TRAPPED SNAPE) against the serenity surrounding them and below, promising perhaps a cradle of happy comfort as their resolution? I think so! On a Happy Isle somewhere nearby!
That sense of balanced outcome stems from the way your action areas fit in so beautifully against the background, the use of light and dark, of soft land(ings) and sea below, while vividly overhead fly your brightly colored Snape and Harry. And let me just note? I love their green vs. red! Also, the way Snape's collar point poking through his hair gives the hint of a devil's pointed ear, to go with his goatee! Yay, Lucifer Snape, falling into the new dawn of brightly irrepressible Harry!
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Erm. That is to say - LURVE. I would totally hoard the comic book this is the perfect cover to, never ever parting with it. Maybe sleeping with it.
What a happy start! I hope everything is this solidly executed and filled with delight!