Bonjour
For your background image
1) Your image is a PNG, its size is 1,9 Mo
turn this image into a JPG file ( JPG 30% ~ 90 ko or JPG 60% ~200ko ). The loading of your page will be faster.
2) On each page of an iWeb site, the background image URL is different so the visitor need to upload a new image per page. If you use the same big image (with the same file URL) for a lot of pages, choose a color for the browser background with the inspector, upload your image to your server, get its URL and paste this kind of code in a HTML snippet widget
http://iweb.debutersurmac.com/bg/testbox.html . My widget (test box) can help you to understand how the code works (replace the image URL by yours).
You can also try this new version of the code (Thanks to Wyodor).
http://dl.dropbox.com/u/3563737/Lycos/Beach.html
Notice that this image will be fixed and covers all the background whatever its size.