DOCBOOK-APPS: Centering Figure Graphics

I don't know if this is a misunderstanding of the DTD, a problem with the 
style sheets, or a failure on my part to correctly customize the style 
sheets.  I am including graphics as figures in DocBook (SGML) 4.1.  An 
example is:
       <figure id="fig04">
         <title>Alternate form of a tabard</title>
         <mediaobject>
           <imageobject>
             <imagedata
               align="center"
               fileref="RegaliaFig04.bmp"
               format="bmp"
               srccredit="Zenobia Napthali">
           </imageobject>
         </mediaobject>
       </figure>

I am processing this with the Modular Docbook Style Sheets (1.60); I have 
not (intentionally) applied any customization to the elements involved here.

However, I would like the figures to be centered on the page, and they are 
showing up left-justified.  What am I doing wrong?

Mark B. Wroth
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