The simplest way to clear out the error would be to just add an empty string as an additional parameter:
.JHTML::_('image', 'administrator/components/com_mycomponent/assets/images/'.$image, '')
Obviously, that's not a great alt tag for the image to have, but it shouldn't have an error anymore.
You could also add it as a parameter to the function but set a default value for it in your function, so you could set it appropriately in the future but still clear the error for all existing uses of the function like so:
public function iconButton( $link, $image, $alt='' ) {
$lang = &JFactory::getLanguage();
$button = '';
if ($lang->isRTL()) {
$button .= '';
} else {
$button .= '';
}
$button .= ''
.'<a href="'.$link.'" target="_blank">'
.JHTML::_('image', 'administrator/components/com_mycomponent/assets/images/'.$image, $alt )
.'</a>'
.'';
$button .= '';
return $button;
}