Ah now I was too quick: I thought it over and we can't ask everybody to switch off the add blocker. Any image url should not be blocked. Why can I ask other images via HTTP and not this one. This is not regular HTTP is it?
And my own research found why: the Highcharts image apparently either has a tracker in the image or Highcharts is seen as a reason to block. An image by myself from my own domain did not raise an issue.
And in addition: how do I get an image over the whole container area?
I found the answer to this one as well: an image on the whole background for a chart by setting the chart fill to none and put a background on the containing div works fine.
As I said: sorry for the successive questions, thanks very much for the rapid response which put me on the right track.
(though a confusing Highcharts image I must say)