Create a file called phpinfo.php in your DocumentRoot
<?php phpinfo(); ?>
Run it from the command line (php phpinfo.php), check for the gd section. If the gd section is missing, you didnt install gd. Now run the same file from the browser (http://domain.com/phpinfo.php) and again check the gd section. My guess is that the browser gd section will be missing or disabled. Did you remember to restart Apache httpd or whatever web server you are using after installing gd?