Taking a screenshot in New Leaf is simple: press L and R at the same time. Boom. You've got a picture. Great! Getting those pictures off your 3DS, while simple, requires a few more steps. Nothing too complicated, though.
The image above, which details the process, is from Nintendo's website -- you can find that here. Putting it in words, basically: take a screenshot. From there, use the 3DS' internet browser, and visit this website.
There, you have to log into either Twitter or Tumblr. Once you do, a menu comes up that allows you to post your pictures; in my experience, Tumblr only allows you to upload one picture at a time. I assume Twitter does the same. Kind of cumbersome, sure, but the pictures aren't so big -- uploading shouldn't take more than a few seconds. Be warned that whatever you upload automatically has the #ACNL hashtag attached to it.
If you don't have a Twitter or Tumblr, the next best thing is to pop out your SD card and put it on your computer somehow -- like, say, by using an SD card reader. Either way, hopefully you're taking plenty of pictures and sharing them with the world: much of the fun in Animal Crossing involves showing other people what you've done.
Now get out there and take some good pictures. Tomorrow, we're giving you a chance to show us your home in New Leaf -- better go tidy up!