To explain the solution, here is the complete steps I made

First I had an HTML page without any background:

Then I added some CSS to set a background image :

body
{
   background:url("img/bg.gif") no-repeat fixed left top #9AE4E8;
}

The result was good :

But I wanted to add a second background image on the right bottom :

So I moved my background declaration for the head part of the html file

html
{
 background:url("img/bg.gif") no-repeat fixed left top #9AE4E8;
}

and I added an other background declaration for the body part

body
{
 background:url("img/logobeachrahier_medium.png") no-repeat fixed right bottom ;
}

The background declared for the body is applied over the html one:

I think it’s Pretty good now.