You are correct, ADA defines an “employer” as any person who is 1) engaged in an industry affecting commerce; 2) employes 15 or more full-time employees each work day; and 3) for at least 20 or more calendar weeks in the year. In the context of physical spaces, ADA would not apply to companies with fewer than 15 employees. However, courts don’t seem to have come to a consensus on what digital compliance really should look like. Because websites can be accessed anywhere in the country, small business owners might potentially face lawsuits in unfavorable jurisdictions. If you are a small business owner, I would recommend consulting with an accessibility lawyer and ask what they recommend.
In some circumstances, longer and more detailed text will be necessary to convey the same meaningful information that other visitors to the website can see. For example, a map showing the locations of neighborhood branches of a city library needs a tag with much more information in text format. In that instance, where the map conveys the locations of several facilities, add a “longdesc” tag that includes a text equivalent description of each location shown on the map – e.g., “City Center Library, 433 N. Main Street, located on North Main Street between 4th Avenue and 5th Avenue.”
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.