In our 8 1/2 years experience in programming websites we found a lot of times search engines and directories do in fact search for the content within the alt tag of the image.
Why you may ask? Not only is it a great alternative for the user if the image does not display on the page, but also for our impaired users, more specifically, our web user who cannot see. Search engines and search directories like to reward us web developers who keep everyone in mind.
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