$desired_width = 500;
$desired_height = 500;

$im = imagecreatefromstring($blobcontents);
$new = imagecreatetruecolor($desired_width, $desired_height);

$x = imagesx($im);
$y = imagesy($im);

imagecopyresampled($new, $im, 0, 0, 0, 0, $desired_width, $desired_height, $x, $y);

imagedestroy($im);

header('Content-type: image/jpeg');
imagejpeg($new, null, 85);