I'm quite desappointed not to have answer.
In order to understand my problem I give you an example of the matter : code below
<?php
header("Content-type: image/png");
$im = imagecreate(200,200);
$black = imagecolorallocate($im, 0,0,0);
$white = imagecolorallocate($im, 255,255,255);
$font="arial.ttf";
imagettftext($im, 18, 45, 100, 100, $white, $font,"Test");
imagepng($im);
imagedestroy($im);
?>
Thanks for your help.