On 2004/07/12 we grew 5 more callout images, but the default value
for %callout-graphics-number-limit% remained 10 in docbook-dsssl
and the new images were not used. So we need to specify our own
value for %callout-graphics-number-limit%.

Note that in docbook-xsl callout.graphics.number.limit is 15 now.
OTOH docbook-dsssl hasn't been updated for 2 years and keeps the
old default.