Because only Internet Explorer displays tooltips using the 'alt' tag, all other browsers are missing out on the display picture description. The easy fix is to also add a 'title' tag to images to the userpics in addidtion to the 'alt' tag.

Line 4457 in moodle\lib\weblib.php just needs to go from:
$output .= "<img class=\"$class\" src=\"$src\" height=\"$size\" width=\"$size\" alt=\"".s($imagealt).'" />';

to:
$output .= "<img class=\"$class\" src=\"$src\" height=\"$size\" width=\"$size\" title=\"".s($imagealt)."\" alt=\"".s($imagealt).'" />';

This is definitely not a big issue but is one that can be very quickly fixed. I don't know where else this might be an issue!

Josh Lukins