I\'m working with DockBook 4.5 and Apache FOP 1.1 on Ubuntu 13.04. The Docbook translation are provided by Ubuntu and FOP was downloaded directly from Apache.
Would
you need to use a formatter tnat supports fo:float.
http://xmlgraphics.apache.org/fop/compliance.html
For an example, it would be like this:
<fo:float float="right">
<fo:block text-align="center"
border="1pt solid gray"
font="bold italic 9pt Helvetica">
<fo:block>
<fo:external-graphic src="url('smile.gif')"/>
</fo:block>
<fo:block>
Fig. 1: A Smiling Face
</fo:block>
</fo:block>
</fo:float>
This builds upon Kevin's answer. The solution is to use a Formatted Object Processor that supports the feature. Apache FOP is feature anemic in some critical areas like floating an image (unless you are Gutenberg working with a printing press).
If you are looking for a free one, then try XEP (http://www.renderx.com/). It works as expected, and it has a free license for non-commercial projects.
For completeness, Sagehill lists a bunch of processors at http://www.sagehill.net/docbookxsl/FOprocessors.html. I tired three of the free ones, and they are all broken. The remaining free ones no longer exist (or their web pages are broken).