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"alt gives you minimal, absolutely essential information; title adds useful information and 
can add flavour; longdesc (to be examined shortly) provides for rich, expressive documentation 
of a visual image." 

Further, he gives an example in the Forms and interaction chapter:
"<input type="image" src="button-6.gif" alt="Continue" name="Continue" id="Continue" 
value="Continue" title="Proceed to checkout" /> 

It&#8217;s redundant, redolent of outdated coding kludges, and vaguely laughable, but legal 
nonetheless. 

(With all five text equivalents present, Lynx displays value; using only one at a time, or 
simply deleting value, my testing shows that Lynx displays alt by preference but no 
alternative text at all if you use only title, id, or name. The details are somewhat academic, 
I know.)" 

Basically, use ALT to describe the function (as Whitney said, a verb), and TITLE for further 
description.

I hope this helps.

Best regards,

Ash Donaldson
User Experience Designer