DOCBOOK: How to center a graphic?

[Gautier HARMEL <harmel@aemiaif.lip6.fr>]

Hello everyone,

I'm a beginner with DocBook.
I use DocBook v3.1 

My pb is that when I do something like that : 
<!DOCTYPE mediaobject PUBLIC "-//OASIS//DTD DocBook V3.1//EN">
<mediaobject>
<imageobject><imagedata fileref="figure/cable.eps" align="center"
format="eps"></imageobject>
<imageobject><imagedata fileref="figure/cable.gif" align="center"
format="gif"></imageobject>
<textobject><phrase>Ethernet Cable</phrase></textobject>
<caption><para>Vue en coupe d'un cable Ethernet</para></caption>
</mediaobject>

I don't have the behavior expected. The figure is always align on the
left, and I'd like to align it on the center of my page.

I compile this file with this command line :
jade -t sgml -d /usr/lib/sgml/stylesheets/nwalsh-modular/html/docbook.dsl presentation.sgml

I'd like to know how to center object. 
Do I have to make my own stylesheet ? And if so, how can I do that ?

Thank you very much for your help !

Gautier Harmel