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Amongst the sizes accepted under the pilot scheme are Banners at 468 x 60 pixels, Leaderboards at 728 x 90, Inline Rectangles 300 x 250 and Skyscrapers 120 x 600. At the bottom of each image ad will be the URL and a Google feedback link.

The ads will have to be below 50K and not be aimed at an over 18 audience - i.e. no nudity or other 'adult' content.

The image ads will work on the same basis as the more traditional text ads: advertisers bid on a particular keyword and websites that show the image ads on their pages will be paid per click and Google promises that the ads will still be contextual. For those web sites that can handle graphical ads, this should be a bonus as - all things being equal - image ads generally do better than text-only ones.

The image ads are unlikely to become as diverse as Google's traditional text based AdWords programme. Anyone can put up an AdWord campaign in minutes using the Google wizard whereas images need a greater degree of separation.

Expect a rash of job opportunities for graphic artists at the search engine marketing companies.