The primary accessibility concerns for the deaf (website should not need speakers), blind (website should be usable without a monitor), and mobility-impaired (keyboard driven — no mouse needed).
Use this magic bit of CSS to hide text.
.hide-text {
text-indent: 100%;
white-space: nowrap;
overflow: hidden;
}
The back-story - from an accessiblity and SEO standpoint, it is nice to be able to have a textual description of an image, especially if that images contains text on it. This way screen readers can speak the contents of that image and search engines can index it well.
http://www.netmagazine.com/features/5-html5-and-aria-design-patterns
The article “Web Accessibility: Tools and Considerations” describes several tools for testing accessibility and a quick overview for coding an ARIA compliant page.
https://addons.mozilla.org/en-US/firefox/addon/wave-toolbar/
https://addons.mozilla.org/en-US/firefox/addon/ainspector-sidebar/