The accessibility movement encourages web sites to be built to allow people with disabilities to view them. For example, one accessibility standard is that all images have "alternate text" and "long descriptions" coded into the HTML. This would be useful for software that reads web pages out loud for blind people. Even if they cannot see your images, the software can read the description of the image out loud.
There are two different guidelines often used when determining whether a site is "accessible": the US Government Section 508 Guidelines and the W3C Web Content Accessibility Guidelines.
Accessible Web Design
Best Practices for Accessible Flash Design
EASI (Equal Access to Software and Information)
How to Make Accessible Web Content Using Microsoft Expression Web - Webaim
Web Accessibility Design Sites and Tools
Trace Center - Designing More Usable Web Sites
Commonwealth of Massachusetts, IT Division, Web Accessibility Standards