DOCBOOK-APPS: sapcing before image

How can I impose spacing before images in print output?

I know I need the FO to be:

<fo:external-graphic space-before.optimum="0.5cm" src="images/blah.png" 
 width="auto" height="auto" content-width="auto" content-height="auto"  
 text-align="center"/>

But I don't know how to put this into my customisation layer, if
I see an example of how to do this it should help me apply the
example to other things I want to do as well.

Regards

ion