centring a picture

[Alex Kleider <akleider@sonic.net>]

Still a beginner, I've become a great fan of docbook, mainly to then 
convert to html.
My current project requires insertion of photos (png, jpeg).
Is there a way to specify that a photo 
(<mediaobject><imageobject><imagedata>)should be aligned in the middle, 
rather than to the left margin of the the page?
Thanks in advance for any help.
alex