Thumbnail Blocking via Sophos Firewall

If you are using Sophos Firewall for Web Protection and you are trying to get search engines such as Google not to return results that are pornographic (including potentially previews for those websites - what I assume you mean by thumbnails) you need to use the SafeSearch feature of those search engines.

SafeSearch can be enforce by the web proxy, go to the web policy you are using and Click Edit.  
If you are using DPI mode, please see this article for steps: community.sophos.com/.../sophos-firewall-safesearch---enforcement-when-using-the-dpi-engine

If you are using Sophos Firewall for Web Protection and you are trying to get search engines such as Google not to return results that are pornographic (including potentially previews for those websites - what I assume you mean by thumbnails) you need to use the SafeSearch feature of those search engines.

SafeSearch can be enforce by the web proxy, go to the web policy you are using and Click Edit.  
If you are using DPI mode, please see this article for steps: community.sophos.com/.../sophos-firewall-safesearch---enforcement-when-using-the-dpi-engine