Background-image

Pls show your site URL.

Try this: in style.css from line 14,
use
background:url("http://sylvainmeret.com/sylvainmeret/wp-content/uploads/2010/01/waterfall3.jpg") 800px -20px no-repeat;
instead of

background-image:url("http://sylvainmeret.com/sylvainmeret/wp-content/uploads/2010/01/waterfall3.jpg");
background-repeat:no-repeat;
background-position: 800px -20px;

In my browsers(firefox, safari and chrome), the image shows up.

Try to refresh your page,
or add above rule at the end of style.css AND refresh the page.

body{
background:url("http://sylvainmeret.com/sylvainmeret/wp-content/uploads/2010/01/waterfall3.jpg") 800px -20px no-repeat;
}