Help! Custom Fields for Lead Image?

I’m simply trying to figure out how to put a title image on the websites front page when a post is the lead article. I assume this is some sort of custom field – what is the name of the field? If not, how do I do it? Is there a list of custom fields just for reference so that I can use them? I’m using the Arras theme, not sure if that helps or not.

Any help would be greatly appreciated. Thanks everyone!