primo:
http://it.php.net/manual/it/function.array-rand.php


secondo:
codice:
function str2img($string){
	header("Content-type: image/jpeg");
	$im = imagecreate(400, 30);
	$white = imagecolorallocate($im, 255, 255, 255);
	$black = imagecolorallocate($im, 0, 0, 0);
	 // Replace path by your own font path
	imagettftext($im, 20, 0, 10, 20, $black, "arial.ttf",$string);
	imagejpeg($im);
	imagedestroy($im);
}
str2img("ciao oronze");
http://it.php.net/manual/it/function.imagettftext.php