imagecreatetruecolor and other image functions return a image reference, which allows you to generate or append imagery to them.
But in order to reference them you need to assign them to a variable,
$myPic = imagecreatetruecolor(100,100);
$myPic is now your reference
imagejpeg($myPic, "myfiles/image.jpg", 100);
will save your created jpeg to a file called image.jpg in a directory called myfiles with quality of 100
a whole mess of goodies to play with:
http://us2.php.net/manual/pl/ref.image.php
just remember, you need to have gd2 library enabled in your server to allow for these functions. if you need help with that let us know