Currently it appears that figjam will only allow users to set a thumbnail image to a file if you copy and past a frame from a figma file. The problem with this is that you cannot make a universally edible header and save it as a component in figma for use across all file types. When you save a component from a figma file and put it into a figjam it loses it ability to be used as a thumbnail image for some reason. Allowing users to save cards as components and stick them into figjam files would help for greater organization within the file navigation menu.