I think if you have a game running or if a specific game supports it, you can. You open the home menu mid-game, make a post and there should be an option to add the screen then!

i got the screenshot plugin while installing aroma and the if you click l, dpad down and a you can get to the plugin menu and change the binding of the screenshots there

wii u feature only, theyre working on adding image upload support for 3ds juxt, hopefully soon!