Yep. I found the root issue. It's caused by that I changed the default setting of "Use CAPTCHA for: " to:

Survey Access/--------/Save & Load

If I changed it back to:
-------/-------/------

Then it would be OK...

Can you please upload your config.php file (xxx out any passwords)? I just want to check that there is no other source for this problem.

Also attach your survey for testing, thanks!

Here it is(with values for those key variables):

// Basic Setup

$databasetype = 'mysql'; // ADOdb database driver - valid values are mysql, odbc_mssql or postgres
$databaselocation = 'localhost'; // Network location of your Database - for odbcmssql use the mssql servername, not localhost or IP
$databasename = 'limesurvey'; // The name of the database that we will create
$databaseuser = 'survey'; // The name of a user with rights to create db (or if db already exists, then rights within that db)
$databasepass = 'xxxxxx'; // Password of db user
$dbprefix = 'lime'; // A global prefix that can be added to all LimeSurvey tables. Use this if you are sharing
// a database with other applications. Suggested prefix is 'lime_'
// File Locations
$rooturl = "http://{$_SERVER['HTTP_HOST']}/limesurvey"; //The root web url for your limesurvey installation (without a trailing slash). The double quotes (") are important.

$rootdir = dirname(FILE); // This is the physical disk location for your limesurvey installation. Normally you don't have to touch this setting.
// If you use IIS then you MUST enter the complete rootdir e.g. : $rootDir='C:\Inetpub\wwwroot\limesurvey'!
// Some IIS installations also require to use forward slashes instead of backslashes, e.g. $rootDir='C:/Inetpub/wwwroot/limesurvey'!
// If you use OS/2 this must be the complete rootdir with FORWARD slashes e.g.: $rootDir='c:/limesurvey';!
// Site Setup
$sitename = 'CDC Survey Engine'; // The official name of the site (appears in the Window title)
$defaultuser = 'admin'; // This is the default username when LimeSurvey is installed
$defaultpass = 'xxxxxx'; // This is the default password for the default user when LimeSurvey is installed

// Email Settings

$siteadminemail = 'Dinny.Wu@my-company.com'; // The default email address of the site administrator
$siteadminbounce = 'Dinny.Wu@my-company.com'; // The default email address used for error notification of sent messages for the site administrator (Return-Path)
$siteadminname = 'Dinny Wu'; // The name of the site administrator

// Company Email Server Connection Settings
$emailmethod = 'smtp';
$emailsmtphost = 'smtp-na.my-company.com';

// Company LDAP services
//$enableLdap = true;

Can you also upload your survey for testing as .csv file?

In the htmlsourcecode the captcha is <img src='/verification.php' />
Seems to me that the $rooturl is not set when generating the page.

If you install limesurvey into a subdirectory the captcha won't get generated.
because /verification.php will be not found.

verification.php is used three times in index.php :
<img id='captchaimage' src='$rooturl/verification.php' />

When I declare $rooturl global in the function buildsurveysession() (which i CANNOT recommend as a proper solution, since I don't know the codebase well enough ) the captcha is working fine.

Thanks for identifying the source of the problem, jelo. This should be fixed soon.

Thank you for your kindly analysis, Jely. It's really the $rooturl variable's problem, which needs to be declared as global.

Anyway, I know where the problem is and I have made a temp workaround for urgent use.

Declaring it global is fine. You should still deactivate notices on a production system.