Making verification code harder to OCR?

I'm using a verification code image to stop automated sign ups, but two hackers seem to be OCRing it. I've looked through the registration script, and there's definitley no security holes. Does anyone have any ideas as to making the image harder to OCR? <?php // seed with microseconds function make_seed() { list($usec, $sec) = explode(' ', microtime()); return (float) $sec + ((float) $usec * 100000); } $seed = make_seed(); mt_srand($seed); $dbh = mysql_connect ("", "", "") or exit; mysql_select_db ("",$dbh) or exit; $authimage = ImageCreate(40,15); $bgnum = mt_rand(1,3); switch($bgnum){ case 1: $white = ImageColorAllocate($authimage, mt_rand(250,255), mt_rand(250,255), mt_rand(250,255)); break; case 2: $green = ImageColorAllocate($authimage, mt_rand(0,5), mt_rand(250,255), mt_rand(0,5)); break; case 3: $yellow = ImageColorAllocate($authimage, mt_rand(250,255), mt_rand(250,255), mt_rand(0,5)); break; } $black = ImageColorAllocate($authimage, mt_rand(0,30), 0, 0); header("Content-type: image/png"); $getcode = mysql_fetch_array(mysql_query("select * from signupcodes where id = '$id'")); imagestring($authimage,mt_rand(4,5),mt_rand(0,5),0,$getcode['code'],$black); imageline($authimage,0,mt_rand(0,15),40,mt_rand(0,15),$black); imageline($authimage,0,mt_rand(0,15),40,mt_rand(0,15),$black); imagepng($authimage); imagedestroy($authimage); ?>