Brooklyn feature image

So I’m new to all these stuff but although I’ve been watching and reading tutorials, searching and trying I can’t seem to be able to do even the simplest modifications. I changed once the main picture in the Brooklyn theme but can’t seem to find out how to do it again https://syff.ca/

Everything seems to be extremely extremely frustrating, we’re a volunteer run small film festival, we cannot pay for any web developers so it’s all on me, so can somebody please please help me solve this issue no matter how simple it seems to you? A huge box appears on a page that I just updated and I just can’t remove it, no matter how hard I try, I’ve been trying to figure it out for over 5 hours and it’s actually starting to drive me crazy… please help!!!! this is the page: https://syff.ca/syff-2016/