How-Tos and Demos
Accessible Graphics
Example: alt Text for Decorative Images and Spacers
The image in this example is the kind that may be added to a page for visual or design interest, but which contains no meaningful information. Such an image may be an entire stand-alone graphic, or it might be something like part of a frame around a more meaningful image, etc. In other words, it's part of the visual design but doesn't carry meaning. The purpose of the alt text for such decorative graphics is to make them "invisible" to screen readers, so that users are not distracted from the page's content.
How to do it
Code example for alt text on decorative images 
Rationale
Graphics provide a satisfying visual experience for users who are sighted. Alt text should contribute to the auditory or tactile experience of users who are visually impaired and others who use text-only displays. Identifying or describing images that provide no meaningful content, such as decorative borders, spacer images, or other "eye-candy," will only impede the access of a visually impaired reader to the content.
The most appropriate textual alternative in these cases is simply to have the screen reader skip the image. In order to do this, provide an empty alt attribute (alt=""). You can't just leave the alt attribute out completely, because (1) screen readers will then read the name of the file that contains the image, and filenames for merely decorative files are usually meaningless; and (2) because the correct HTML requires an alt attribute for every <img> element.
How it sounds with the JAWS screen reader
[Screen reader is silent, skipping over images with empty alt attributes]
JAWS transcript
[Screen reader is silent, skipping over images with empty alt attributes]