A lot of people seem to be having this problem.
users aren't able to get captcha to verify them, the image appears ok, but verification fails.
I was getting this problem too. and I think Ive found the answer.
After trying to register a few pseudo-users myself, using a bunch of different browsers I realised that there was a page caching problem with the new user page.
If a user visits the registration page to check it out and then navigates away for whatever reason they get the cached captcha image from the previous visit - which obviously isn't the right one
The same thing happens if the page verification fails, because they got a detail wrong such as not enough chars in the info box.
I only noticed this because one particular image was vaguely memorable - if you have the problem try adding a new user, remember the image code, then navigate away from the page (not just doing a refresh), then go back.
Same number? Your problem is that the browser is caching the image, rather than getting the newly generated one.
most likely this is a htaccess 'mod_expires' problem - even more likely if you have dolphin in a subdirectory and something creating dynamic content like drupal or joomla in the www directory.
The answer, for me anyway, was to add this code to the bottom of my .htaccess file - the one in the main Dolphin directory, where all the dolphin files and folders are kept.
# this sets the http file to expire from cache
<IfModule mod_expires.c>
# Enable expiry.
ExpiresActive On
# Cache all files for 5 seconds after access
ExpiresDefault "access plus 5 seconds"
</IfModule>
The above code needs to be added right at the bottom of the code in the .htaccess file - make sure that it doesn't split ANY existing blocks of code.
In theory the page with the image should be having a 'dont cache' directive sent with it - but if you've got any higher level .htaccess files, they may be screwing it up - so this fix should work for directories below the dolphin installation folder.
while I was looking I did spot that the simg.php script that calls for the captcha image looks a bit odd, it seems to be adding a img/gif header to the .png file it outputs - also the header that it creates may be deprecated for xhtml.
