I did worked with the plugin YAPB (also possible with standard WordPress thumbnail but needs bit of code moding) and used this setting in the arrays of JW player.
It might be not fully automated but you can control your introduction image.
<?php if ($post->image): ?>
'image': '<?php echo $post->image->getThumbnailHref(array('w=400','h=276','q=95','zc=1','fltr[]=usm|30|0.5|3')) ?>',
<?php else: ?>
'image': '<?php echo get_stylesheet_directory(); ?>/img/no-image-video.jpg',
<?php endif; ?>