i'm trying to paste a picture i took on my device to give it to someone but i don't know how?

well, the way you're phrasing that makes it seem somewhat un-sylestia related but:
you need to upload the picture to the internet some way (eg, with imgur, discord, deviantart, photobucket) then get the image url of that image (eg. https://i.imgur.com/G7AD4Jc.jpg - they'll end in .jpg, .gif, or .png usually)- here's a guide on the ways different browsers present that; though the demonstration of it is using sylestia images the concept of how to get the image url is the same
(though if you upload to google pictures you could just make a folder with it in and send that, if you both have discord, then you can just send it to them there, deviantart you can just link the page etc etc)

but that's the gist.
to put images into a sylestia post you just put it in img tags
[*img]https://i.imgur.com/G7AD4Jc.jpg[*/img] without the *'s breaking it

makes the image