This is not an answer, but a followup and a workaround:
By turning off abstract url handling (in acs/parameters/$server.tcl):
--------------------------------------------------
# enable abstract url handling?
# For an explanation, see: doc/abstract-url.html
ns_param EnableAbstractURLsP 0
-------------------------------------------
and restarting the webserver, the images appeared.
So the problem seems to have something to do with abstract url handling, or some component that abstract url handling relies on.
I would still be interested to know if anyone has opinions whether this is possibly an open-ACS buglet, or perhaps a mod_aolserver issue ... or how to fix it. 😊
Thanks again for any suggestions,
Os