When AdSense runs out of inspiration (i.e.: no targeted ads are available), it displays public service ads by default. You can choose to replace this with a custom ad of your own choosing. To do this, you have to put a HTML page somewhere and change the alternate ads settings for the appropriate ad unit:

Instead of the public service ads, I wanted to serve a banner ad to promote our beautiful family vacation home. I created this image a few years ago for an AdWords campaign started by my uncle because he wanted to reach more potential vacation goers.

The 728x79 banner image

I put put up a page with the image at http://www.bigsmoke.us/schuilplaats-banner-ad/. The HTML is very simple:

<html>
<body>
</body>
</html>

Although I put the alternate ad up over a year ago, I was prompted to blog about it because two months ago, for the first time, I suddenly saw the ad actually being serviced on my Dutch HardwoodWiki: