Ads between viewed posts

Hello,

I run an image based site and I each image is linked to a random post. So, you view an image, click the image it will bring up a different image (that is its own unique post). What I want to do is ad an ad after say 3/5 clicks.

So a user clicks the image and is delivered another random post, this continues after 3/5 posts brings up just an ad page.

Sorry if it doesnt make sense, but any help or plugin suggestion would be greatly appreciated.

Thank you