People with disabilities depend on the web way more then we do. There are devices to convert web content to formats that disabled can read. For example, there are braille converters for blind people. However images cannot be converted into such formats. Hence designers are asked to make thier sites accessible to people with disabilities by providing alternet textual descriptions of all image and video content that they use on thier pages. People with disabilities may not be able to view the images but at least will know what the images are.