For an image that is simply a bit-map of text (because you want to use special graphical font-effects or other transformations that would be difficult or impossible using style sheets), provide the text equivalent, e.g.
<IMG SRC="wai.gif" ALT="Web Accessibility Initiative">
Where a special markup language for symbols exists - such as MathML - use that instead of a simple text equivalent.  See the example for Checkpoint 3.1 for more details.
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