Thanks
for your contributions.
You are right in your remark about
the path to images, Wolfgang. I was not using any
\setupexternalfigure[location=default]
instruction. With your formulations the figure is recovered
without problem.
Unfortunately the results of your rewrite
of the macro are not so satisfactory. Surely it is more *ConTeXt*ual
than mine but it fails to fulfill the main purpose of the exercice: it
happily prints fleurons in blank pages, as running your tests with 6
lines demonstrates. In such a circumstance it should produce just two
pages with six lines each, whereas it outputs 4 pages, the two
additional ones containing only ornaments.