Okay, so for the CSS, you just need to use your browser.
Right click on the avatars, choose Inspect (Element), then you should see something like
As you can see, it selected the image you right-clicked on and choose inspect element, you want to remove the whole column, so navigate up the DOM tree to the td.topic-poster, that is the class you want to target, which means you end up having
td.topic-poster { display: none}
You should probably put .topic-list on it too, so it becomes
.topic-list td.topic-poster { display: none}