Thanks for your fast response, nice one.
Sorry it's not of great help. I tried to explain above what is the cause of this. Personally I have to disable for the time being hence I cannot post a link.
I think the basic facts are that - Facebook's Like Button will scan your page for the first available image. Of course - if you have a post with no images - the first image that will appear is the Captcha.
So we need to find a way to:
1. Force facebook NOT to use the Captcha image (on a normal page)
2. Force facebook to use a default image (on a page with no images)
There is a plug in that solves problem 2. But it only seems to work if the Captcha is not on that page.
I hope that helps but sorry I can't post a link as I have had to disable for the time being.
The FB Lint tool may help you trace it as you can enter any URL there and it will show you what image FB will use.