PS Vita allows users to easily capture direct-feed screenshots from the system. However, Nintendo 3DS is currently missing that feature. A new app released by Nintendo in Japan will introduce that functionality--but only in select titles.
According to Polygon, the screenshot tool allows players to take screenshots while playing Animal Crossing: New Leaf (or the Japan-only Tomodachi Collection). You'll then be able to share those images on Twitter or Tumblr.
So far, only Nintendo of Japan has announced the tool, but we'd be surprised if this didn't make its way Stateside in some shape or another.