Hoi Hans,
Try the following to increase your page length:
<div id="content" style="height:1300px">
Try to increase the number to i.e 1700px.
Also,
Add the following line at the bottom of your page (before </body>):
<script type="text/javascript"> document.getElementById("bg_clouds").style.height = document.getElementById("bg_clouds").clientHeight + 220 + 'px'; </script>
</body>
With changing the 220+ 'px', you're sure the page will display correctly in regards to the background.
Hope this helps!!
Cheers,
Arthur
PS:
Doe de groetjes aan m'n broer. Hij woont in Westkapelle