Vertical Images is the latest international beta test being run by Google AdSense. Vertical images displays a related generic image within the ad unit, which then links to a results page similar to the results page seen when clicking on an Ad Links keyword.
This test could potentially be seen on all publisher sites utilizing specific ad units. While the test initially launched earlier this week in only skyscrapers, they are now being seen in leaderboards, meaning the initial testing is likely successful. However, they are still only being displayed on a limited number of page views.
I have more details, including a screenshot of an example ad unit, at JenSense.
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