In html, the use of ID's and classes are different. For what you are doing you will need to use the class identifier, not the ID identifier.
.banner
{
background-image:url(img/content-bottom.png);
}
<div class="banner"></div>
The alternate way is:
#banner
{
background-image:url(img/content-bottom.png);
}
<div id="banner"></div>
And the easiest way to center your image is to use a display block and auto margin, don't forget to set the image height and width in css:
div.banner
{
display: block;
margin-left: auto;
margin-right: auto;
background-image:url(img/content-bottom.png);
height: 48px;
width: 48px; <!--Set image height and width-->
}
<div class="banner">
</div>
Hope that helps.
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