One of the simplest ways to make your website more accessible is to use an alt attribute in your image tags. It's amazing to me how many people forget to use this simple attribute. In fact, now, if you want to write valid XHTML or HTML5, the alt attribute is required for the img tag. And yet people still don't do it.
The alt attribute is an attribute of the "img" tag and is meant to be an alternative for non-visual browsers when they come across images. This means, that the text is meant to be used when the image is not visible on the page. Instead, what is displayed (or read) is the alternative text. Many browsers also display the alt text when the customer rests their mouse on the image. This means that text in this tag should be clear and easy to read and not create a huge pop-up nightmare for any reader pausing their mouse on your page.
This attribute is supported in all major web browsers.
The required alt attribute specifies an alternate text for an image, if the image cannot be displayed. This attribute provides alternative information for an image if a user for some reason cannot view it (because of slow connection, an error in the src attribute, or if the user uses a screen reader).
Note: Internet Explorer (prior version 9) displays the value of the alt attribute as a tool-tip, when mousing over the image. This is NOT the correct behavior, according to the HTML specification.