Posters (and most settings) for JW Player are HIGHLY dependant on the version of the player, and the method with which you are using it. I've spent literally hundreds of hours working with it, and still need to check with their forums when I need to tinker.
Probably not the answer you'd like, but a starting point might be at this Poster Image question on their support forum..
We use playlists for all our videos, regardless of how many videos are actually being played, and here's a sample playlist XML that we use. Again, it all depends on how you are using JWPlayer, but this might get you started.
<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0" xmlns:media="http://search.yahoo.com/mrss/" xmlns:jwplayer="http://developer.longtailvideo.com/trac/">
<title>Sample Bitrate Switching Playlist</title>
<description>Description of your video.</description>
<title>My Video Title</title>
<description>Description of this playlist item</description>
<media:content bitrate="2400" width="1000" url="yourvideo_1500.mp4"/>
<media:content bitrate="800" width="640" url="yourvideo_900.mp4"/>
<media:content bitrate="400" width="320" url="yourvideo_425.mp4"/>
<media:thumbnail type="image/png" url="http://www.yoursite.cm/yourposter.png"/>