How-Tos and Demos
Accessible Graphics
Example: alt Text for Functional Graphics
This example shows the image of a blue arrow to be used as a link to the next page in a sequence
How to do it
Code example for alt text on a functional image 
Rationale
The primary purpose of alt text for functional graphics (such as graphical links, graphical buttons, etc.) is to identify the purpose of the graphic for users who can't see it, rather than to describe the image itself. In this example, a blue arrow serves as a link to another page in a sequence. Thus, alt text saying, "Next page" is more useful than alt text that says "Blue arrow," because it tells users what will happen when they activate the link.
The same principle applies to alt text for graphical buttons such as a 
button on a search form (<input> elements of type="image") and image-map hotspots (<area> elements)
How it sounds with the JAWS screen reader
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JAWS transcript
[Link graphic Next page]