Image created with GD does not appear

Hello, I generate an image with GD in the file generate_image.php <?php // generate_image.php require('config.php') require('functions.php'); require('secure.php'); header('Content-Type: image/png'); $im = imagecreate(15, 15); // generation of the image // ... imagepng($im); imagedestroy($im); ?> But when I call this page in an image tag like this : <img src="generate_image.php" border="0"> The image is not found (I have the small red cross). If I drop the 3 lines (require) at the beginning of the page then it works fine. The image is well displayed. So I tought it was a problem with the headers, but I have verified, the 3 included files doesn't send any header. That's very strange because if I have the 3 includes files in the code and then I go direct to the location of the file generate_file.php (and so I don't call it with a tag), then the image is well displayed. Did you have an idea why the imaeg is not displayed when I call it with an img tag ? Thanks a lot