Adding featured image to posts created by a plugin

The quickest and dirtiest way to do this is to add one line to the insert_posts() function, just before the $imported++; line:

        }

        add_post_meta( $post_id, 'raw_import_data', json_encode( $twitter_raw ) );

        // NEW LINE (change 1001 to the image ID):
        set_post_thumbnail( $post_id, 1001 );

        $imported++;
    }

This will obviously disappear if you ever update the plugin. There aren’t any hooks built into the plugin to add your desired functionality, so a more rigorous solution would have to periodically query for posts with, say, the keyring_service post metadata set, check for any that don’t have the thumbnail set, and then set it to the default.