It won't block wikipedia by default because that's a legit site, if you want to block wikipedia you can add a filter against it to stop access from your connection.
Also, when it comes to image searching on Google, it won't block Google showing the cached image, but if you try to go directly to the adult website it is hosted on, that is when you'll encounter the block (if the website is known.)
Summary: * WebSafe will not block cached images in search engine results * Websafe will not block images on known safe sites, even if they include some nudity
For making youtube searches safe: ensure all your computers and devices have their default web browser logged into a Google account with parental features enabled.
Edit: typo fix.
- - - - - - Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.