I dont use Dreamweaver but if you go into code view you should be able to edit the CSS directly and center a background image withthis CSS;
Code:
html {
background:url('path/to/img.jpg') 50% 50% no-repeat;
}
The html selector puts the image as part of the base web page background, but you can substitute in other selectors. For example, you could center the image in the background of a specific div using;
Code:
#mydiv {
background:url('path/to/img.jpg') 50% 50% no-repeat;
}
The div markup would be <div id="mydiv"></div>
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