Aquí está mi propuesta.
Yo iba a hacer lo mismo que tu, pero mejor busque un script que lo hiciera automáticamente... y la solución fue un script para hacer thumbs automáticamente, te lo digo por el uso que le he dado, te las redimensiona, por ejemplo:
Tu tienes una imagen de 800x750 y claro la quieres mas chica, al thumb le especificas por ejemplo x=400 y=400 esto significa que la redimensionara usando maximo 400 de ancho y 400 de alto, este es su limite, la redimensiona de tal modo que se vea bien usando como limite maximo 400 de ancho y alto.
Pruebalo, aquí te dejo el script e indicaciones, el script se llama Thumbnail Image Generator 1.3
En un images.class.php guarda esto:
Código PHP:
<?
class Zubrag_image {
var $save_to_file = true;
var $image_type = -1;
var $quality = 100;
var $max_x = 100;
var $max_y = 100;
var $cut_x = 0;
var $cut_y = 0;
function SaveImage($im, $filename) {
$res = null;
// ImageGIF is not included into some GD2 releases, so it might not work
// output png if gifs are not supported
if(($this->image_type == 1) && !function_exists('imagegif')) $this->image_type = 3;
switch ($this->image_type) {
case 1:
if ($this->save_to_file) {
$res = ImageGIF($im,$filename);
}
else {
header("Content-type: image/gif");
$res = ImageGIF($im);
}
break;
case 2:
if ($this->save_to_file) {
$res = ImageJPEG($im,$filename,$this->quality);
}
else {
header("Content-type: image/jpeg");
$res = ImageJPEG($im, NULL, $this->quality);
}
break;
case 3:
if (PHP_VERSION >= '5.1.2') {
// Convert to PNG quality.
// PNG quality: 0 (best quality, bigger file) to 9 (worst quality, smaller file)
$quality = 9 - min( round($this->quality / 10), 9 );
if ($this->save_to_file) {
$res = ImagePNG($im, $filename, $quality);
}
else {
header("Content-type: image/png");
$res = ImagePNG($im, NULL, $quality);
}
}
else {
if ($this->save_to_file) {
$res = ImagePNG($im, $filename);
}
else {
header("Content-type: image/png");
$res = ImagePNG($im);
}
}
break;
}
return $res;
}
function ImageCreateFromType($type,$filename) {
$im = null;
switch ($type) {
case 1:
$im = ImageCreateFromGif($filename);
break;
case 2:
$im = ImageCreateFromJpeg($filename);
break;
case 3:
$im = ImageCreateFromPNG($filename);
break;
}
return $im;
}
// generate thumb from image and save it
function GenerateThumbFile($from_name, $to_name) {
// if src is URL then download file first
$temp = false;
if (substr($from_name,0,7) == 'http://') {
$tmpfname = tempnam("tmp/", "TmP-");
$temp = @fopen($tmpfname, "w");
if ($temp) {
@fwrite($temp, @file_get_contents($from_name)) or die("Cannot download image");
@fclose($temp);
$from_name = $tmpfname;
}
else {
die("Cannot create temp file");
}
}
// check if file exists
if (!file_exists($from_name)) die("Source image does not exist!");
// get source image size (width/height/type)
// orig_img_type 1 = GIF, 2 = JPG, 3 = PNG
list($orig_x, $orig_y, $orig_img_type, $img_sizes) = @GetImageSize($from_name);
// cut image if specified by user
if ($this->cut_x > 0) $orig_x = min($this->cut_x, $orig_x);
if ($this->cut_y > 0) $orig_y = min($this->cut_y, $orig_y);
// should we override thumb image type?
$this->image_type = ($this->image_type != -1 ? $this->image_type : $orig_img_type);
// check for allowed image types
if ($orig_img_type < 1 or $orig_img_type > 3) die("Image type not supported");
if ($orig_x > $this->max_x or $orig_y > $this->max_y) {
// resize
$per_x = $orig_x / $this->max_x;
$per_y = $orig_y / $this->max_y;
if ($per_y > $per_x) {
$this->max_x = $orig_x / $per_y;
}
else {
$this->max_y = $orig_y / $per_x;
}
}
else {
// keep original sizes, i.e. just copy
if ($this->save_to_file) {
@copy($from_name, $to_name);
}
else {
switch ($this->image_type) {
case 1:
header("Content-type: image/gif");
readfile($from_name);
break;
case 2:
header("Content-type: image/jpeg");
readfile($from_name);
break;
case 3:
header("Content-type: image/png");
readfile($from_name);
break;
}
}
return;
}
if ($this->image_type == 1) {
// should use this function for gifs (gifs are palette images)
$ni = imagecreate($this->max_x, $this->max_y);
}
else {
// Create a new true color image
$ni = ImageCreateTrueColor($this->max_x,$this->max_y);
}
// Fill image with white background (255,255,255)
$white = imagecolorallocate($ni, 255, 255, 255);
imagefilledrectangle( $ni, 0, 0, $this->max_x, $this->max_y, $white);
// Create a new image from source file
$im = $this->ImageCreateFromType($orig_img_type,$from_name);
// Copy the palette from one image to another
imagepalettecopy($ni,$im);
// Copy and resize part of an image with resampling
imagecopyresampled(
$ni, $im, // destination, source
0, 0, 0, 0, // dstX, dstY, srcX, srcY
$this->max_x, $this->max_y, // dstW, dstH
$orig_x, $orig_y); // srcW, srcH
// save thumb file
$this->SaveImage($ni, $to_name);
if($temp) {
unlink($tmpfname); // this removes the file
}
}
}
?>
En thumb.php guarda esto
Código PHP:
<?
// Parameters:
// src - path to source image
// dest - path to thumb (where to save it)
// x - max width
// y - max height
// q - quality (applicable only to JPG, 1 to 100, 100 - best)
// t - thumb type. "-1" - same as source, 1 = GIF, 2 = JPG, 3 = PNG
// f - save to file (1) or output to browser (0).
// Sample usage:
// 1. save thumb on server
// http://www.zubrag.com/thumb.php?src=test.jpg&dest=thumb.jpg&x=100&y=50
// 2. output thumb to browser
// http://www.zubrag.com/thumb.php?src=test.jpg&x=50&y=50&f=0
// Below are default values (if parameter is not passed)
// save to file (true) or output to browser (false)
$save_to_file = true;
// Quality for JPEG and PNG.
// 0 (worst quality, smaller file) to 100 (best quality, bigger file)
// Note: PNG quality is only supported starting PHP 5.1.2
$image_quality = 100;
// resulting image type (1 = GIF, 2 = JPG, 3 = PNG)
// enter code of the image type if you want override it
// or set it to -1 to determine automatically
$image_type = -1;
// maximum thumb side size
$max_x = 100;
$max_y = 100;
// cut image before resizing. Set to 0 to skip this.
$cut_x = 0;
$cut_y = 0;
// Folder where source images are stored (thumbnails will be generated from these images).
// MUST end with slash.
$images_folder = '../imagenes/articulos/';
// Folder to save thumbnails, full path from the root folder, MUST end with slash.
// Only needed if you save generated thumbnails on the server.
// Sample for windows: c:/wwwroot/thumbs/
// Sample for unix/linux: /home/site.com/htdocs/thumbs/
$thumbs_folder = '/www/thumbs/';
///////////////////////////////////////////////////
/////////////// DO NOT EDIT BELOW
///////////////////////////////////////////////////
$to_name = '';
if (isset($_REQUEST['f'])) {
$save_to_file = intval($_REQUEST['f']) == 1;
}
if (isset($_REQUEST['src'])) {
$from_name = urldecode($_REQUEST['src']);
}
else {
die("Source file name must be specified.");
}
if (isset($_REQUEST['dest'])) {
$to_name = urldecode($_REQUEST['dest']);
}
else if ($save_to_file) {
die("Thumbnail file name must be specified.");
}
if (isset($_REQUEST['q'])) {
$image_quality = intval($_REQUEST['q']);
}
if (isset($_REQUEST['t'])) {
$image_type = intval($_REQUEST['t']);
}
if (isset($_REQUEST['x'])) {
$max_x = intval($_REQUEST['x']);
}
if (isset($_REQUEST['y'])) {
$max_y = intval($_REQUEST['y']);
}
if (!file_exists($images_folder)) die('Images folder does not exist (update $images_folder in the script)');
if ($save_to_file && !file_exists($thumbs_folder)) die('Thumbnails folder does not exist (update $thumbs_folder in the script)');
// Allocate all necessary memory for the image.
// Special thanks to Alecos for providing the code.
ini_set('memory_limit', '-1');
// include image processing code
include('image.class.php');
$img = new Zubrag_image;
// initialize
$img->max_x = $max_x;
$img->max_y = $max_y;
$img->cut_x = $cut_x;
$img->cut_y = $cut_y;
$img->quality = $image_quality;
$img->save_to_file = $save_to_file;
$img->image_type = $image_type;
// generate thumbnail
$img->GenerateThumbFile($images_folder . $from_name, $thumbs_folder . $to_name);
?>
La forma de uso es así:
<img src="thumb.php?src=ejemplo.jpg&x=400&y=400&f=0" border="0" />
Puedes cambiar el valor de X y Y, si deseas más info, checa la pagina
http://www.zubrag.com/scripts/thumbnail-generator.php
Saludos :D
$_FILES y libreria GD 
