The secret to being able to count how many times an advert is displayed is to have it on your own website, not on the site where you're advertising.
When I discussed how to measure the clickthroughs from banner ads last week, one point which was raised was: "But I'm not running the banner ads just for clickthroughs, they're also for branding, so how do I measure how many times they get seen?" Good question. Again, never take the figures given to you by the website owner. Set up your advert so that you can count the number of times it was shown.
Before I explain how to do this, do remember that the figure you get will be the number of times the advert was shown on a page, which might not necessarily be as many times as it was seen. If the advert is low down on the page, for example, it might not be seen unless the person looking at the page scrolled down to reveal it. However, it will count as having been shown. So do bear that in mind.
The secret to being able to count how many times an advert is displayed is to have it on your own website, not on the site where you're advertising. What the viewer on the other site will see is an advert "served up" from your site. That's not hard to do, and the website where you're advertising will be used to this, even if they don't actively encourage it. Any designer who's put together the advert for you will be able to arrange for the image to be stored on a page on your (or any other friendly) site, and called up using what's known as an "IFRAME". Don't forget you'll also be in charge of where the image should click through to, so you'll need to tag that appropriately.
If your advert designer (or the website where you're advertising) reckons there are any problems with this, consider working with someone else. Ten years ago, when I was editor of Engineeringtalk, up to half of the dozens of banner adverts we ran at any one time were being provided this way. It's an old technique, and one which everyone should now use.