Adding a border around a DocBook <imageobject> in PDF output

Hello,

I have some screenshots to include as part of a user manual I'm compiling with DocBook, and several are on white backgrounds and contain mainly text, so doesn't mix well with other text layout on the page.  I tried to find out how to add a border by regarding DocBook XSL documentation, specifically:

- http://www.sagehill.net/docbookxsl/BordersAndShading.html

- http://www.sagehill.net/docbookxsl/Graphics.html

..and the best I could do was following the second link, add a border around the <figure> element (so it not only goes around the image, but also around the <title>, leaving a lot of whitespace around the image).

I then tried working out how add borders using the first link instead, so that I'd only outline the image itself, but none of <mediaobject>, <imageobject>, or <imagedata> appear in the list of elements that recognize the attribute-sets referred to there.  A little further on, it says "you will want to find out where the outer fo:block is output..." but I don't know what I'm looking for based on that advice, nor how to find it, nor what to do with it (I'm a bit worried that I might accidentally clobber something too by matching the template in a higher-precedence customization layer).

Thanks,
Christopher