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:
H89: Using the title attribute to provide context-sensitive help:
and previous WebmasterWorld discussion...
The Title Attribute: