alt attribute is the first step for supporting accessibility on the web.
WebAIM (Web Accessibility in Mind) has an article with several tips, Designing for Screen Reader Compatibility. Among the many tips is:
Screen readers will read the alternative text of images, if alt text is present. JAWS precedes the alternative text with the word "graphic." If the image is a link, JAWS precedes the alternative text with "graphic link."
W3C has defined a standard called WAI-ARIA, which further enhances accessibility on the web:
WAI-ARIA, the Accessible Rich Internet Applications Suite, defines a way to make Web content and Web applications more accessible to people with disabilities.
You can find the 1.0 spec here: http://www.w3.org/TR/wai-aria/
It goes into further detail on how to improve your markup to define roles for various regions within your site. From the spec's Abstract:
Accessibility of web content requires semantic information about widgets, structures, and behaviors, in order to allow assistive technologies to convey appropriate information to persons with disabilities. This specification provides an ontology of roles, states, and properties that define accessible user interface elements and can be used to improve the accessibility and interoperability of web content and applications. These semantics are designed to allow an author to properly convey user interface behaviors and structural information to assistive technologies in document-level markup.
For example - you can associate a label with a textbox, and place both of those inside a form region. This provides accessibility software with a much better idea of your site's structure, and can communicate that to the user better.