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.
To Checkpoints for Guideline 1.Next slide: Example for Checkpoint 1.1c