Japan Gets 3DS Image Sharing (Screenshot) Tool
Unfortunately, only two games is currently supported by this app which are Animal Crossing New Leaf and Tomadachi Collection: New Life.
Also, this will not be a separate eShop app. Instead, it's an online service which can be accessed at https://i.nintendo.net and must be accessed via the Nintendo 3DS browser.
Screenshots that you take can be shared to Social Network Services which currently seem to include Twitter and Tumblr (how about Facebook!?).
No word for a Western release yet, but that is most certainly, a sure thing to come. We'll also probably see more games using this feature in the future!