crea immagine - librerie gd

[maxtn]

Non capisco come mai non riesco a creare delle immagini, le librerie gd sono installate.
Se provo questo non mi fa vedere niente, neanche errori....

// create a 100*30 image
$im = imagecreatetruecolor(100, 30);

// white background and blue text
$bg = imagecolorallocate($im, 255, 255, 255);
$textcolor = imagecolorallocate($im, 0, 0, 255);

// write the string at the top left
imagestring($im, 5, 0, 0, "Hello world!", $textcolor);

// output the image
header("Content-type: image/jpeg");
imagejpeg($im); 

gd
GD Support enabled
GD Version bundled (2.0.28 compatible)
FreeType Support enabled
FreeType Linkage with freetype
GIF Read Support enabled
GIF Create Support enabled
PNG Support enabled
WBMP Support enabled
XBM Support enabled