Pretty sure they aren't fanmade. I found this old blog post on the official website detailing different services available with KH Mobile:
http://blog.jp.square-enix.com/kingdom/2011/04/post-10.html
You'll notice one with an image of Aerith and a portrait on a similar landscape as the wedding picture we saw earlier (Disney Castle is in the background, but the terrain is slightly different). Those are identified as cellphone wallpapers (待ち受け; short for 待受画像
, and it describes the process of getting to them as going to Point Art Gallery->Wallpapers->FF CHARA->Aerith Illustration 1 or Point Art Gallery->Wallpapers->AVATAR KINGDOM->At the flower field.
"Point Art Gallery" is ポイントアートギャラリー, by the way.
Here are some more examples:
http://blog.jp.square-enix.com/kingdom/images/20110520.jpg
The Cloud vs. Sephiroth image and the one of Sora handing Kairi a heart-shaped balloon are wallpapers ("machiuke"; 待ち受け
; the other images are decome (デコメ
, short for "decomeeru" (デコメール
-- or "decoration mail." They're basically cellphone based stationary where you can add your own text.
There were examples of those in that blog post I linked to earlier as well:
http://blog.jp.square-enix.com/kingdom/2011/04/post-10.html
In this case, the image of Aerith in the flower field and the "THANKS MOTHER" image are stationary templates while the little Moogle and Heartless pictographs are little touches of personalization that you can add to them. Obviously, you can edit the text on them as well.
As near as I can see, there's no feature for creating a wallpaper, though. The closest to that you can do is placing your avatar on a different background that you've unlocked, but -- as you can see at the KH Wiki article for the game -- those backgrounds are identified with "Vignette Room" and all feature a cylindrical platform in the center of whatever KH location they represent:
http://kingdomhearts.wikia.com/wiki/Kingdom_Hearts_Mobile
I think you can also customize your profile backgrounds with clothes and items that you've purchased, as in this image:
http://blog-imgs-34.fc2.com/e/l/w/elwi/12917583.jpg
That seems to be the extent of what you can "create," though.
So, yeah, I'm fairly well convinced that the Valentine's Day image and the wedding image are not fanmade. That said, with the KH Mobile service discontinued since April 30 of 2012, I have approximately zero confidence of finding them on an official website.