I've been looking at this for a few days now, and while I can't explain every aspect of the picture, the overall mood of something that was once pure and cherishable is now being corrupted really jumps out at me. The dead birds, the leg and hair becoming tainted by some sort of black, amorphous entity; it all seems to reflect something good coming under attack and losing, very gradually. Even the red tones around the girl's face and neck seem to imply that something incredibly sinister is assaulting her and the environment around her.
I don't think I've looked into any sort of Newgrounds work of art like this is an incredibly long time. Absolutely spectacular.