The image came from Pottermore by changing the URL of an existing spell's wand movement to the one you wanted to see.
The revision that added the wand movement to the Harry Potter wiki page was done on the 2nd of August, 2012 by user Hunnie Bunn (see the previous revision to confirm).
On the talk page archives there is an entry where Hunnie Bunn appears to be replying to an unsigned comment or edit that mentions changing a portion of the URL of an image to 'killing-curse' in order to see the wand movement:
This image is extracted from Pottermore. Simply open the image of any curse and change it to killing-curse.
http://i44.tinypic.com/2wcpsns.jpg
I'm not entirely sure what you mean by "open the image of any curse", but I'm going to take your word for it and add that to the hand movement part of the infobox. Thanks for your help, but next time please sign it.
User:Hunnie Bunn 21:13, July 17, 2012 (UTC)
The user who was trying to explain the process wasn't extremely clear, but I read it as: "the wand movement was initially found by opening an existing wand movement's image, and changing the URL" - from /assets/expelliarmus.png to /assets/killing-curse.png for example.
A few months later Hunnie Bunn posted a blog post titled Hidden Pottermore Images. The blog post primarily focusses on whether or not the images are canon, but it also mentions a "glitch" that allowed people to view unreleased wand movements.
1337star (The user who originally uploaded the Killing Curse wand movement image to the wiki incidentally) commented on the post, and linked the Hover Charm talk page, where an anonymous user explained the "glitch":
If you add that spell to yout (sic) favourites on Pottermore and view it in your "Favourite Spells" tab, you can see that the hand movement of the Wingardium Leviosa spell is the same.
1337star says in the information only recently came to their attention. The entry to the Hover Charm talk page was made on the 18th September 2012 and 1337star uploaded the image for the Killing Curse wand movement on the 17th of May 2012, so they could not have known about this "glitch" when they uploaded the image.
The comment on the image (which has not changed since the original upload) further drives the point home:
This image is currently unused on Pottermore, and thus may not be suitable for use on articles.
Taking into account that 1337star had already uploaded the image for the wand movement prior to knowing about the "glitch", and the fact that other wand movements were found to be legitimately from Pottermore by looking at the spell's image URL on the favourites page, makes me firmly believe the image came from Pottermore and was retrieved by changing the spell's name in the image URL.