From Seth Bienek, a procedure for centering content on a page both vertically and horizontally - this appears to require a 4.0 browser (as it makes use of the  <DIV> tag...

1. Add the image you wish to use for this to the Assets View:
• Go to the Assets View
• Select “New File Asset” from the “Assets” menu
• Select the image file in question and check the “Always Publish” checkbox
• Click on OK
2. On the page you wish to center the content on, Right-Click (Control-Click if you´re on a Mac) in the Layout area and select “Layout HTML” from the pop-up menu that appears
3. Enter the following code in the “Beginning of Body” field:

   <div align=”center”> <center> <table border=”0” cellpadding=”0” cellspacing=”0”  width=”100%” height=”100%”> <tr> <td width=”100%” height=”100%” valign=”middle”  align=”center”> <img src=”yourimagehere.gif” width=xxx height=yyy> </td> </tr> </table> </center> </div>

   where  yourimagehere.gif is the image you wish to center (with the appropriate path to it), XXX is the width of the image and YYY is the height of the image
4. Click the OK button