While some might be tempted to do this if you understand anything about how Google indexes websites, someone who has thousands of pages on their site would lose rankings. The price of making your site fully accessible depends on how large your website is. Making the best decision for your specific situation is best done by consulting a professional.
Courts have essentially taken three positions when approached with this issue. Some courts take the position that the ADA applies to all commercial sites because the law was meant to protect disabled individuals from having a more difficult time than able-bodied individuals from doing business. That is why the website Scribd was unable to get a case summarily dismissed and ended up settling.
The words in the tag should be more than a description. They should provide a text equivalent of the image. In other words, the tag should include the same meaningful information that other users obtain by looking at the image. In the example of the mayor’s picture, adding an “alt” tag with the words “Photograph of Mayor Jane Smith” provides a meaningful description.