• CNet, Thursday, March 5, 2009 11:30 AM

Google added expandable banner units to AdSense on Wednesday, but unlike most expandable ads, they don't grow larger when you mouse over them. Rather, users have to click on the smaller ad to see the larger one.

Google probably chose to do it this way because many users find expandable ads annoying, but CNet's Steven Musil says the format could be valuable to both advertisers and customers when used to show movie or video game trailers, or other images that require greater emphasis on detail.

The search giant said advertisers are automatically eligible to use the expandable ads if they've added AdSense code and enabled image ads in their site's source code. The program is currently only available to specific advertisers in the U.S. that can bid on specific ad placement. Earnings are based on whether the advertisers uses cost-per-click or cost-per-impression ads. CPC ads generate more earnings when a use visits the advertisers landing page and not when a user simply clicks to expand the ad.

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