Image CAPTCHA does not work with Konqueror

setup: Drupal 6, CAPTCHA 6.x-1.x-dev, image CAPTCHA on comment form

The image does not show up in like 19 out of 20 tries with the Konqueror web browser.
Problem seems that konqueror requests the image two times, which does not work because the CAPTCHA image can only be generated once (by design). I tried several cache control directives, but haven't found a solution.

Problem also exists for CAPTCHA 5.x-3.x, but to a much less degree, only like 1 out of 20 tries fails.
The difference at first sight is that in the 5.x-3.x version the $_SESSION data is not yet cleared in the second request in most cases, while in the 6.x-1.x version the $_SESSION data is cleared in most cases.

ugly problem and difficult to debug :(