[TIP] If you want to share a game screenshot on social media simply tap {HOME} and open the browser. Then type {http://theimageshare.com} and on the website click {search}. Select the screen and {upload}. This generates a QR Code to open with your phone! Boom, you're welcome.
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You can also use the Rosalina Menu to take a screenshot and then transfer it to your phone via FTP or by putting your 3DS's SD card in your phone and then copying the screenshots
No need to install anything. Works like magic. The website needs to be http and not https. For ease of use just add the website to your favourites, that way you don't need to manually type the address every time.
Last note: This only works if you have a game (or an app) running for obvious reasons. You can't do this while on your homescreen or miiverse.