since no-one is helping with a definitive answer I will share my knowledge on how I would go about this. I think you need to use javascript and cookies, basically you would write a cookie to their PC the first time they visit the site and show the image, everytime they load the page it would check if the cookie exists, and if it does would not show the image as you assume they saw it last time when it wrote the cookie.

I did this sorta thing at uni but it was 4 years ago and it would take me sometime to work out again and frankly I can't be bothered but if you read through these tutorials on javascript you might be able to achieve what your try to do.

http://codepunk.hardwar.org.uk/ajs.htm lessons 26 onwards.

Hope this helps a little.

Stuart