Use this instead:
<script>
jwplayer('video-front').setup({
height: 540,
width: 980,
playlist: [{
image: "<?php echo $base_url; ?>/sites/default/files/video-front/thumbnail-223_0002.png",
sources: [
{ file: "<?php echo $base_url; ?>/sites/default/files/video-front/1380575675.mp4" },
{ file: "<?php echo $base_url; ?>/sites/default/files/video-front/1380575748.webm" },
{ file: "<?php echo $base_url; ?>/sites/default/files/video-front/1380575648.flv" }
]
}]
});
</script>
The image needs to be inside of the playlist block. That should fix it.