Extensive Guide to Forum Syntax
ToC for ‘How to picture…’ there.
Even without trust lvl 3, there are whitelisted sites that can be used for posting images. Over at Memes of the Storm - #2962 by Rafaelfras-1457 you can find examples of pics linked from reddit.
i usually use <img src=[http://sampleurl.linktopic.format> w/ optional <height =[value]. i usually post screen caps ported to imgur, others use discord links, and there’s probably other options too.