I usually take the approach of selecting the element with right click and inspect, then on the inspect console I copy the JS path, document.querySelector("#post_1 > div > div.topic-avatar > div > a > img") for your avatar for example.
Then try it on the console with document.querySelector and document.querySelectorAll to check if i got the right thing.
Also it might be that you are trying to get the elements without them properly loading to the DOM so you might want to implement some implicit or explicit waiting strategies.