Both captcha options YTK provides should be inaccessible to you in the control panel of the program. So, that said, just use the captcha link to launch your default browser to get your captcha challenge code. Messenger 8.0 and 8.1 are still supported by YTK fully but it wasn't until Messenger version 9.0 that Yahoo! integrated the inline captcha into the chat screen window.

Why would you want to use 8.1 out of curiosity? It's very dated but I know many like to use it because it has much less bloat than 9.0 and newer. Anyway, there's ways to make even the newest version 11.5 less bloated by disabling the plug-ins from loading by default and through Messenger's preferences itself. Newer versions contain fixes to the code and updates to the core services that Yahoo! Messenger provides so I personally wouldn't recommend using 8.0, 8.1, or 9.0 at all anymore. If you'd like a limited protocol version then you can use Auto-Reconnect with either YMSGv15 or version 102 - (which I'd recommend at the moment for best boot protection).

You'll have less features to use in the client but you'll have better boot protection because of this. With the next two anti-boot systems you'll be harder to boot and when Dual-Mode is completely added to and tested in YTK then you won't have any feature limitations whatsoever and you'll remain logged into the chat room even if you are booted/disconnected (which will automatically be resumed).