cgsi_convert does not replace the class (and alt, as reported by adrien jaures
in 11377) parameters. Can be fixed by adding:

(This is meant as a suggestion)

case 'alt':
    $value = trim($value);
    if($value) $sxe[$key] = trim($value);
    break;
 case 'class':
    $value = trim($value);
    if ($value && substr($value,0,1) == "+") {
        $sxe[$key] = trim($sxe[$key] . " " . substr($value,1));
    } elseif ($value) {
        $sxe[$key] = $value;
    }
    break;

@ line # 976 of lib/class.cgsi_utils.php of version 1.22.2

{cgsi_convert class="+new-class"} would then append the class name(s), while
omitting the + sign will replace the img class attribute.