Blank Boxes when viewing site from fresh browser

Hello, I am having a strange problem with lightbox2 on my website: http://nolanwebsolutions.co.uk/portfolio

basically what is happening is if you goto the website for the first time and goto the portfolio page. When you click on one of the thumbnails it is just opening a blank white box. Rather than showing the imagecache Image I have set.

If however you log in it works and if you log in and then log out again it works from then on in that browser.

Have you any idea what is going on?

Any help would be greatly appreciated.
Kind Regards,
Ryan