The tep_image() function is not documented but the source code is fairly straightforward:
function tep_image($src, $alt = '', $width = '', $height = '', $parameters = '') {
//[...]
if (tep_not_null($parameters)) $image .= ' ' . $parameters;
$image .= ' />';
return $image;
}
The fifth parameter gets injected as-is right before the end tag. That's all you need.