Callouts don't appear with Docbook PDF

[PeterRobertson]

I have a small example that uses docbook callouts. If I transform in
the Oxygen environment using Docbook HTML, I get the expected output
(having created the missing directory images/callouts and copying in the
various .png files).

However, if I use Docbook PDF, the callout bugs disappear - everything
else is correct. If I set the parameter callout.graphics to 0, I get small
numbers for the bugs as expected (after changing the color parameter in
callout.xsl to "blue" rather than white, as someone suggested in a
message I found).

Yet again, I have run out of ideas to try to make this work. Does anyone
know the magic ritual needed?

Is there anything in this system that simply works as the books describe?

[PeterRobertson]

It turns out that you have to enable PNG processing explicitly. Without this, the references to the bugs are silently ignored.