According to Creating an Accessible Presence for the Lodging Industry by the American Hotel & Lodging Association, “Many businesses don’t understand that the law actually requires them to make their websites accessible to individuals with disabilities. There is a common misperception that the Americans with Disabilities Act and similar state and local laws only deal with physical buildings and facilities, so-called “brick and mortar” establishments, and that non-building-related business operations—such as websites—are not covered by these laws. Nothing could be further from the truth—at least with respect to websites that have some sort of connection, or “nexus,” to physical places of business.”
UPDATE: Since writing this post in August 2017, several important changes have taken place in the laws regarding ADA compliance for websites. On December 26, 2017, the Department of Justice announced that they have withdrawn the Obama-era Advance Notice of Proposed Rulemaking mentioned in this article which intended to require ADA website compliance. The DOJ’s withdrawal announcement stated, “The Department will continue to assess whether specific technical standards are necessary and appropriate to assist covered entities with complying with the ADA.”

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.