I think should be for folder art but you need to rename to possible number like 1 so that it is placed first before others. The first art in your folder will appear as folder art.

But when I try to rename to (folder.jpg or HD Movies.jpg) as same as my folder name, the art does not appear. In fact the the movie "300" art appears as my folder art. So, I try to rename to something like 1 then it becomes the 1st list so eventually the folder art will take the 1st in its list.