Hey. This a coloring of a piece by Geotastic on Deviantart that was done by an anonymous party on 4Chan. Not taking any credit for this piece aside from proposing the coloring to the Anon.

You can find the original picture (and Geo's account here: http://fav.me/d9177e1

As for any official backstory, you'd have to ask Geo. However, if I had to give some context, it'd be simple.

Takes place after the movie. A new personality island has sprung up in Riley's head, and Joy's decided to check it out first-hand.

It's ABDL Island, of course.

Joy seems to be pretty enthusiastic about the new island. The complementary change of wardrobe looks adorable on her, not to mention the fact that she's amazed they made diapers that big.

Food's not bad either (bottles for everyone in this case), and she appears to have come across a stuffed companion that she swears she vaguely remembers the name of...

Regardless, here she is, toddling around her new environment without a care in the world. Although, considering her bottle intake thus far, she may find herself in need of a diaper change relatively soon...

(And, yes, I am aware the coloring's off on Bing Bong.)