Inkscape SVG: Download
No-effects version: Download
From Season 3, Episode 13 - Magical Mystery Cure.
You may notice that the resolution on this image is a little smaller than what I normally upload. Inkscape choked to death when I tried to render this at 6k pixels wide, and the file size of the PNG was approaching 5 megabytes when it died completely. I hope this is large enough for whatever anyone wants to do with it, but if anybody needs a higher resolution version please let me know and I'll try again.
Full 6k wide resolution now uploaded.
I also left the swirl effects on a separate layer, so if anyone needs a copy with just Twilight that can also easily be done. (Update: Check the links at the top of this description.) Updated 02/23/2013: Fixed a small fill error in the left foreleg, and uploaded a full resolution version.