One of the most basic techniques of web accessibility is adding alternative texts to images. It is possibly the single biggest thing you can do to make your site more accessible. In fact, it is the very first item listed in the WCAG 2.0 guidelines:

WCAG 2.0 Guideline 1.1.1 - "All non-text content that is presented to the user has a text alternative that serves the equivalent purpose."  

The most common way of meeting this guideline is adding an alt attribute to the image tag: