<style type="text/css">
.header
{
width: 900px;
height: 300px;
margin: 0 auto;
background-image: url(/img1.png);
}
.img
{
width: 300px;
height: 300px;
background-image: url(/img2.png);
float: right;
}
</style>
<div class="header">
<div class="img"></div>
</div>