Whilst images are an important aspect of the Web, not everyone can experience them. For example, people who are partially-sighted may not be able to see images very well and people who are blind may not be able to see them at all. Some users may disable images to reduce bandwidth and search engines, such as Google, will only index the text on a webpage, therefore any information conveyed by an image will not be processed.
To address this, it is important that all images links have a text alternative that adequately describes the image. Alternative text acts as a fallback for people who, for whatever reason, cannot see the image. It means that people who are blind or partially-sighted can use assistive technology, such as a screen reader or screen magnifier, to read the text alternative of an image link, in much the same way they would read other text content on a webpage. Users who have disabled images will see the text alternative in place of an image, and search engines will be able to process the text alternative and properly index the webpage.
To add alternative text to an image link, use the "alt" attribute (commonly, but incorrectly, called an "alt tag") of the <img> element.
//The alt attribute is used to provide a description of the image link
<a href="http://www.news.com"><img src="logo.jpg" alt="Logo of the News.com network"></a>