Take a picture, using the shoulder buttons.
Go on the internet browser on your 3DS.
Go to the Bookmarks.
Click Nintendo 3DS Bookmarks tab.
Click the first option, Nintendo 3DS Image Share.
Log in to either Facebook, Twitter or Tumblr.
Scroll down a little and select browse.
Choose the photo you want to upload.
(Optional) Add a comment to go with your post.
Click on post.
Save the picture directly from whatever social media site you posted it on!
That's the easiest way for me, since I usually use this site on mobile.