Thanx for the answer, ken.
I just tried Epiphany 2.14.3 (GNOME Web Browser) and there the CAPTCHA was displayed!
But in Iceweasel (the Debian Firefox) it´s still not there. And if i remember correctly i already had this problem some months ago (with firefox 1.5) but just ignored it that time.
Isn´t it strange that it doesn´t work with firefox here? I can´t imagine that i´m the only one using firefox (on Debian). And I checked my firefox preferences - pics from image.captchas.net are NOT blocked.
So the problem still exists but at least I have a workaround.
jere