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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.
I think that since elements of harmony are powerful artifacts, Celestia and Luna used them and they are alicorns, and elements gain they power from bond between those who use them, then conclusion is simple, Celestia and Luna had to pass some sort of challenges to create or unlock elements of harmony, and they turned into Alicorns, now Twilight thanks to bond with her friends, most powerful artifacts in the world, and her own spell has turned herself into Alicorn while Celestia watched ( Celestia guided Twilight indirectly all the time ).
So yea, I agree that Twilight died and was reformed as Alicorn, but I disagree with idea that Celestia turned her into Alicorn, because it would mean she can turn all ponies into Alicorns and it is far to easy and make no sense, I prefer theory that it were elements who played main role in Twilight transformation ( but elements are as strong as bonds of those who wear them ), Starswilr was brilliant mage who created spell what can change destiny, but he did not found any friends and not unlocked elements, and so he failed to become Alicorn ( I bet Celestia tried to send him to old royal sister castle to unlock elements and he failed ).
I think that elements of harmony were main part in Twilight, Celestia and Luna transformation ( while Cadance transformation was result of crystal heart ), Starswirl could not unlock elements because he had no friends.
He was interested in learning, gaining knowledge, experimenting, creating spells ( he wanted to study, research, gain power, very ambitious pony ).
For a powerful wizard who wish to learn and gain fame, he was probably very loyal to the crown, like Archmage or something, but he was also independent ( while Twilight Sparkle is very faithful to her mentor and blindly loyal, ready to do anything for princess and Equestria, risking her own life for others, Starswirl would rather do it out of duty or if ordered to, not because he care ).
Twilight is Lawful Good, simple.
Applejack is Lawful Good
RainbowDash is Chaotic Good.
Discord is Chaotic Neutral ( he is not evil, he just want to have some fun ).
Fluttershy is Good neutral.
PinkiePie is Chaotic Good.
Rarity is Lawful neutral ( for element of generousity, she seems kind of selfish and arrogant, she is lawful but she seems neutral to me ).
Celestia: Lawful Good ( she may be a troll teacher and is very manipulative, but she mean well, many call her lazy, but in season 3 finale we learned that she is just have blind faith in her student, and the fact that she take so many rick and never help I would blame for her being a bit stupid ).
Luna: very hard to tell.
Nightmare Moon: Lawful evil ( I think she does not want to kill any pony or else she would easy kill guards what attacked her and mane 6 instead of scaring them off, so she is not stupid enough to kill entire planet with ethereal night, all she wanted is love and respect, and you wont get it from corpses, so I bet she had plan to keep her subjects alive, though from Celestia probably did not knew it, yet she is villain and I can not make her Lawful neutral ).
Chrysalis: Lawful evil or lawful neutral ( sure, she invaded Canterlot, but she did it to feed her subjects, we do not know story between ponies and changeling's, ponies are xenophobic often so I can quess that they not let changelings join they society, with is why changelings had to steal love, but Chrysalis did not killed Twilight or Cadance, so I can not decide do she should be Lawful neutral or Lawful evil, base on politics really ).
Sombra: Lawful evil or Chaotic evil ( tyrant who enslaved others with fear, lawful evil for sure, though he may be chaotic evil since he tried to use fear to control his subjects, but I think as tyrant he is more lawful then chaotic ), also, I think Sombra was never Unicorn, he had to be some sort of evil dark force who took over empire and used crystal heart to gain physical body, that is also why he could not be killed, because I do not buy that Unicorn with dark magic would be almost as strong as 2 Alicorns and immortal, what kind of Unicorn can turn into shadow fog, I think autors of the story made mistake by giving him physical body in the first place, it would be fine if he was just evil fog to begin with, then no one would complain that he is terrible villain.
Shinning Armor: Lawful good ( I would say Lawful neutral considering how badly he threat Twilight on the wedding, but I think he just tend to be stupid sometime and still good in heart ).
Cadance: Lawful good ( lets be honest, Equestria is full of lawful good characters, and Cadance is on the top of it ).
Trixie: Chaotic neutral ( I though she would be lawful evil, but she seems to be just a arrogant show performer to me, there is nothing evil about her ).
Gildia: Chaotic Neutral ( she is not evil, she is just jerk ).
CMC: Chaotic Good ( was there any doubt ? )
Diamond Tiara: Lawful evil ( she may be a bully, but come on, she was ready to blackmail CMC in her young age, if it is not a road to evil, then what is ? ).
Silver Spoon: Lawful neutral ( I do not think she is bully, more of flank kisser, if it was not for Diamond Tiara, she would turn into nice character ).
Derpy: Chaotic neutral or Chaotic good ( I think chaotic good suit her more ).
Dragons from Spike episode: Chaotic neutral or chaotic evil ( they wanted to kill phoenix before he was born, but I guess that dragons are just violent by nature ).
Spike: Neutral Neutral ( he seems to jump in all directions all the time, so I would say he is very neutral, also, I do not buy dragon core business, after all he was not raised by dragon, dragons proved to be jerks, and even if Applejack saved him from Timberwolf, did not Twilight saved Spike many times ? and do I need to add that Spike biggest fear is being thrown away from Twilight, why would he ever want to leave her willingly, that episode with dragon code in season 3 is full of honest and Spike is out of character most of the time ).
Sorry for long answer, it seems I mentioned more then just main characters.