On this site, any post without a featured image specifically set (such as this one) displays the default floral 'K' insignia anywhere on the site where a thumbnail is displayed. I looked around the web and none of the solutions I found worked. I eventually figured out that adding the following to my theme's functions.php file did the trick:

/**
* Default post thumbnail image.
*
*/

function mk_filter_thumbnail_id( $thumbnail_id, $post = null ) {
     if ( ! $thumbnail_id ) {
          $thumbnail_id = 289; //id of default featured image
     }
    return $thumbnail_id;
}

if (! is_admin() ) { //don't do this on admin pages, it sometimes overwrites thumbnails in the back-end view of the media library
add_filter( 'post_thumbnail_id', 'mk_filter_thumbnail_id', 20, 5 );
}

By the way, just writing so I'll remember it, the third and fourth parameters of add_filter() set a priority so multiple filters can run before or after each other, and the number of parameters the function should expect. WordPress's whole "filter" functionality is an odd way of calling functions, although I suppose it's a practical if roundabout way of hooking into existing functions to add your own code modifications.