If you REALLY want to remove it - not good practice - then I think it's these lines in includes/functions/html_output.php
124 - 130.
PHP Code:// alt is added to the img tag even if it is null to prevent browsers from outputting
// the image filename as default
$image = '<img src="' . zen_output_string($src) . '" alt="' . zen_output_string($alt) . '"';
if (zen_not_null($alt)) {
$image .= ' title=" ' . zen_output_string($alt) . ' "';
}



