In the v4.0.4 beta version of this plugin there is a bug with the way the poster image is generated into the RSS feed.
If an image has been specified on the tag then this image should be generated into the RSS feed. However, in the update to v4 Beta a bug was introduced where the image URL is taken from the global options and not the specific tag options.
In my case the global option for "image" is empty, so I ended up with a blank SRC parameter in the RSS feed.
The fix is to change line 151 in flash-video-player.php as follows:
$rss_output .= '<img src="' . $saved_options['File Properties']['image']['v'] . '" />';
$rss_output .= '<img src="' . $inline_options['image'] . '" />';