I think the easiest way is to put your sd card into a computer (or other device with an sd card slot), tap on the
icon when making a post, then upload an image from the sd card.
If you can't read your sd card, you can go to an image hosting site like imgur on your 3ds web browser and upload the images, then when making a post tap the
, then the
from the dropdown menu, and manually type in the image url.