あるライブラリと一部機能のサブセット互換のサムネイル生成ライブラリ（これくらいちがってたらGPL違反にならないですかね？？？）参考：http://www.white-hat-web-design.co.uk/blog/resizing-images-with-php/

ThumbnailImageMaker.php

<?php

/**
 * ThumbnailImageMaker
 *
 * require GD
 *
 * @author     Junichi Ishida <uzulla@himitsukichi.com>
 * @license    MIT License
 * @version    v0.0.1
 * @link       https://gist.github.com/uzulla/9645443
 */

class ThumbnailImageMaker
{
    private $image_resource;
    private $width;
    private $height;
    public $image_type;

    public function load($file_path)
    {
        $image_info = getimagesize($file_path);
        if ($image_info === FALSE) return FALSE;
        $this->image_type = $image_info[2];

        if ($this->image_type === IMAGETYPE_JPEG) {
            $create_result = $this->setImageResource(imagecreatefromjpeg($file_path));
        } elseif ($this->image_type === IMAGETYPE_GIF) {
            $create_result = $this->setImageResource(imagecreatefromgif($file_path));
        } elseif ($this->image_type === IMAGETYPE_PNG) {
            $create_result = $this->setImageResource(imagecreatefrompng($file_path));
        } else {
            return FALSE;
        }
        if ($create_result === FALSE) return FALSE;

        return TRUE;
    }

    private function setImageResource($image_resource)
    {
        if ($image_resource === FALSE) return FALSE;

        $this->image_resource = $image_resource;

        $this->width = imagesx($this->image_resource);
        $this->height = imagesy($this->image_resource);
        if ($this->width === FALSE || $this->height === FALSE) return FALSE;

        return TRUE;
    }

    public function resize($to_width, $to_height, $allow_expand = TRUE)
    {
        if (!$allow_expand && $to_width >= $this->width && $to_height >= $this->height) return TRUE;

        $new_image = imagecreatetruecolor($to_width, $to_height);
        imagecopyresampled(
            $new_image,
            $this->image_resource,
            0, 0,
            0, 0,
            $to_width,
            $to_height,
            $this->width,
            $this->height
        );
        return $this->setImageResource($new_image);
    }

    public function resizeToHeight($to_height, $allow_expand = TRUE)
    {
        if (!$allow_expand && $to_height >= $this->height) return TRUE;
        $aspect_ratio = $to_height / $this->height;
        $to_width = $this->width * $aspect_ratio;
        return $this->resize($to_width, $to_height);
    }

    public function resizeToWidth($to_width, $allow_expand = TRUE)
    {
        if (!$allow_expand && $to_width >= $this->width) return TRUE;
        $aspect_ratio = $to_width / $this->width;
        $to_height = $this->height * $aspect_ratio;
        return $this->resize($to_width, $to_height);
    }

    function save($filename, $image_type = IMAGETYPE_JPEG, $jpeg_compression = 75)
    {
        if ($image_type === IMAGETYPE_JPEG) {
            $write_result = imagejpeg($this->image_resource, $filename, $jpeg_compression);
        } elseif ($image_type === IMAGETYPE_GIF) {
            $write_result = imagegif($this->image_resource, $filename);
        } elseif ($image_type === IMAGETYPE_PNG) {
            $write_result = imagepng($this->image_resource, $filename);
        } else {
            return FALSE;
        }
        if ($write_result === FALSE) return FALSE;

        return TRUE;
    }

}

ThumbnailImageMakerSample.php

<?php
require "./ThumbnailImageMaker.php";
$t = new ThumbnailImageMaker();

$t->load('sample.jpg');
$t->resize(10, 10, false);
$t->save('out_10x10.jpg', $t->image_type, 80);

$t->load('sample.jpg');
$t->resizeToWidth(50, false);
$t->save('out_w50.jpg', $t->image_type, 80);

$t->load('sample.jpg');
$t->resizeToHeight(50, false);
$t->save('out_h50.jpg', $t->image_type, 80);

$t->load('sample.jpg');
$t->resize(1000, 1000, false);
$t->save('out_1000x1000.jpg', $t->image_type, 80);

$t->load('sample.jpg');
$t->resize(1000, 1000, true);
$t->save('out_1000x1000.jpg', $t->image_type, 80);