phranque - 8:05 am on Sep 1, 2011 (gmt 0)

"alt" is required by the validator (and by human decency, unless it's just a decoration)

when it is a decoration with no semantic meaning, the alt attribute should be used with a null value.
this will make the image "disappear" for most text-based browsers, screen readers and other such user agents.

these w3c-recommended accessibility techniques may provide some useful guidance:

H37: Using alt attributes on img elements:
http://www.w3.org/TR/2008/NOTE-WCAG20-TECHS-20081211/html#H37

H89: Using the title attribute to provide context-sensitive help:
http://www.w3.org/TR/2008/NOTE-WCAG20-TECHS-20081211/html#H89

and previous WebmasterWorld discussion...
The Title Attribute:
http://www.webmasterworld.com/accessibility_usability/3597552.htm [webmasterworld.com]