New button in the top-left corner, give the following options:
Change Theme
Screen Brightness
Power-Saving Mode
Nintendo 3DS Image Share
System Settings
Clicking Nintendo 3DS Image Share opens up the web browser to a page where you can sign into Twitter or Facebook and post any screenshots you've taken. Screenshots can now be taken with the Y button on the home screen. Pressing Y+Up on the D-pad takes a picture of the top screen, Y+Down on the dpad takes a picture of the lower one. I assume they're full quality from the framebuffer, but I haven't checked.
EDIT: CANNOT BE USED IN-GAME. It only takes pictures of the home screen itself, NOT games, even when software is suspended