The spam filter for comments is working now, but when I posted the first comment it didn't appear, so I tried to re-post but got a message saying the comment was already there.

The other trouble I sometimes have with this type of system is that the words are sometimes difficult to read. You can refresh the image, so it only a minor irritation.

Not long ago I was playing around with comment spam filters for Wordpress and found something called JS SpamBlock (v1.4). It is invisible to users if they have a browser that can handle javascript, otherwise they just have to type in a number. It seems to be 99.9% effective.