The United States of America has succeeded  to publish enforce a law on the development of Internet Web pages.
The web paged of all corporations organizations ... etc. must comply to standards that will allow disabled or Blind people to read the material published on the Web.

To quote Cynthia D. Waddell, JD ADA Coordinator:
" Universal Design calls for the development of information systems flexible enough to accommodate the needs of the broadest range of users of computers and telecommunications equipment, regardless of age or disability."

United States Department of Justice Policy Ruling, 9/9/96: ADA Accessibility Requirements Apply to Internet Web Pages 10 NDLR 240

The Americans with Disabilities Act (ADA) requires covered entities to furnish appropriate auxiliary aids and services where necessary to ensure effective communication with individuals with disabilities, unless doing so would result in a fundamental alteration to the program or service or in an undue burden. See 28 C.F.R. 36.303; 28 C.F.R. 35.160. Auxiliary aids include taped texts, Brailled materials, large print materials, captioning and other methods of making audio and visual media available to people with disabilities.

Specifically addressing the needs of people with visual disabilities, this policy ruling points out that providing a text format rather than a graphical format assures accessibility to the Internet for individuals using screen readers.

Comment:
What the law  is saying is if you have images on a web site the Disabled (Blind)  person cant see the image and if that person is using a screen reader or a talking browser and you have used images in your navigator the disabled person cant navigate through the site , this is what the law is referring to as an
"undue burden".

This calls for special consideration when developing web pages and to develop web pages that must comply to certain Disability Standards.

For more information

Standards for Electronic and Information Technology: An Overview (published December, 2000).
http://www.access-board.gov/sec508/summary.htm
Implements the accessibility requirement of the 1998 amendment to Section 508, Technology Access, of the Rehabilitation Act of 1973.

The 16 technical standards regarding Web-based Intranet and Internet Information and Applications that detail the technical and functional performance criteria for accessible Web pages are contained in Section 1194.22 of the abovementioned legislative document.
http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Web