Section 1194.22 Web-based Internet information and applications: (a) A text equivalent for every non-text element shall be provided (e.g., via "alt", "longdesc", or in element content).

To meet accessibility guidelines, the header images (when supported by a theme) need to be given user-editable alt tags.

You can start with adding a text field in Administration -> Appearance -> Header that lets user input alternate text for the header image.

Trac ticket: http://core.trac.wordpress.org/ticket/15926

Hint: custom-header.php