Wait, I'll try to post pic, though I don't know how
If you're on PC then simply open this thread in your browser, scroll down to Quick reply box and check for an icon in the given tools which says "insert picture" and select "upload from PC", not URL, in the pop up box. And click "select files", go to the screenshot's location (copy it from phone to your PC) and click upload. That's it! Note: To click a screenshot on your phone press "volume up+power/lock button simultaneously". It'll be saved in SD card/Phone storage > Pictures > Screenshots.
If you are on phone only, then do what Pankaj said. Or do what I said but in Edge on your phone using Desktop mode. But it'll be a tough task as Edge is really annoying when it comes to desktop mode browsing. The page might suddenly reload or Edge may crash itself.