It's tagged on Pixiv as an animated png; it might've been a mis-tag or a a failed upload on the artist's side, since I can't see the animation on Pixiv either.
Isn't that somewhat akin to saying that a certain wall in a certain dungeon dissapears if you leave your game on for three days or something?
... and you'd expect "animation" to have some actual animation, not just "Blip! Skirt's gone!" Shizuha's "turning of the leaves" power would make some sense in context with it if her skirt just gradually faded away...
