Captcha image doesn't show up sometimes - need instructions for debugging

[mr5]

1.5.8
Everything works for most of the users. However several people have complained that they do not see captcha image at registration time, they see a red cross instead. This happened on both IE and FF for various releases.
As the users are in different countries I need a clear advice on what should I ask them to do for debugging the problem in their browser or with their server definitions. The only thing I do know is to ask them to clear the cache of the browser first.
Could someone put here a step by step instructions on how to debug this problem. I would really appreciate this. Are there any special requirements for server and browser?

[mr5]

The url in question is http://eja.pri.ee/Gallery2/
However, as I said, most of the clients had no problems with captcha, and the question is what to ask from those who had. Thanks.

[mr5]

I have some progress. The user, who couldn't register because he didn't see captcha text (saw red x), had his main language estonian. Thus his text was in estonian. When we changed his main language to UK English (and text was in English) everything was OK (the captcha was visible).
Any ideas how to tuckle this further? Thanks

[mr5]

I enabled debugging.
User went back to estonian language. Captcha was still invisible, but this time there was a bigger placeholder with  a big cross.
From the trace I can see
/include/functions.inc.php

    * Notice line 184: Undefined index: dbcharset


(but this is displayed always with my session as well)
and I can see
SESSION :
------------------
Array
(
    [php_captcha] => 6df524610bcda0a2e5f0719f59a57866
)

Does this mean that the captcha text was sent? And then the problem is that it is just not displayed for some reason in case of estonian (or any other language different from English?)?

Thanks

[Αndré]

Captcha works as expected on my testbed in English and Estonian language. I can confirm, that it doesn't work on your gallery with the Estonian language file:
- http://eja.pri.ee/Gallery2/captcha.php?lang=estonian
but with the English language file:
- http://eja.pri.ee/Gallery2/captcha.php?lang=english

You should perform an upgrade to cpg1.5.10, maybe your problem is already fixed with the up-to-date release. Especially make sure to update (= replace) the Estonian language file.

[mr5]

Upgraded to 1.5.10. Everything is OK. Thanks
Have a great New Year!