经测试，效果不错，jpg 尤佳：

<?php
define('WATERMARK_OVERLAY_IMAGE', $_SERVER['DOCUMENT_ROOT'] .'/public/assets/logo.png');/*
* $img 是图片的绝对路径
* 用法：watermark($_SERVER['DOCUMENT_ROOT'] .'/public/images/bg.jpg');
*/
function watermark($img)
{ob_start();// Set content typeheader('Content-type: image/jpg');// Cerate watermark$wm = imagecreatefrompng(WATERMARK_OVERLAY_IMAGE);// getting dimensions of watermark image$wm_size = getimagesize(WATERMARK_OVERLAY_IMAGE);// Create image$image = imagecreatefromjpeg($img);// Get image size$size = getimagesize($img);// placing the watermark center$dest_x = ($size[0] - $wm_size[0])/2;  $dest_y = ($size[1] - $wm_size[1])/2;// blending the images togetherimagealphablending($image, true);imagealphablending($wm, true); // creating the new imageimagecopy($image, $wm, $dest_x, $dest_y, 0, 0, $wm_size[0] , $wm_size[1]);// Send image to the browser imagejpeg($image, null, 100);//Save image in cache// destroy image and watermakimagedestroy($image);imagedestroy($wm);$output = ob_get_contents();ob_end_clean();unlink($img);$size = strlen($output);     $ftp  = @fopen($img, "a");    fwrite($ftp,$output);     fclose($ftp);
}

 2. png2jpg

You need to create a fresh image with a white (or whatever you want) background and copy the none-transparent pixels from the png to that image:

function png2jpg($originalFile, $outputFile, $quality)
{$source = imagecreatefrompng($originalFile);$image = imagecreatetruecolor(imagesx($source), imagesy($source));$white = imagecolorallocate($image, 255, 255, 255);imagefill($image, 0, 0, $white);imagecopy($image, $source, 0, 0, 0, 0, imagesx($image), imagesy($image));imagejpeg($image, $outputFile, $quality);imagedestroy($image);imagedestroy($source);
}

3. gif2jpg

For those interested in my solution, I simply used the built in GD PHP functions. I have little experience dealing with gif files so I was expecting this to be difficult. The fact of the matter is the CodeIgniter Image_lib and extended library (Which I never got to work properly) is overkill for this.

$image = imagecreatefromgif($path_to_gif_image);
imagejpeg($image, $output_path_with_jpg_extension);

4. Resizing images with PHP

The following script will easily allow you to resize images using PHP and the GD library. If you’re looking to resize uploaded images or easily generate thumbnails give it a try

Update: Looking to resize transparent PNG’s and GIF’s? We’ve updated our original code, take a look at http://www.white-hat-web-design.co.uk/blog/retaining-transparency-with-php-image-resizing/

Usage

Save the code from the ‘the code’ section below as SimpleImage.php and take a look at the following examples of how to use the script.

The first example below will load a file named picture.jpg resize it to 250 pixels wide and 400 pixels high and resave it as picture2.jpg

<?phpinclude('SimpleImage.php');$image = new SimpleImage();$image->load('picture.jpg');$image->resize(250,400);$image->save('picture2.jpg');
?>

<?phpinclude('SimpleImage.php');$image = new SimpleImage();$image->load('picture.jpg');$image->resizeToWidth(250);$image->save('picture2.jpg');
?>

<?phpinclude('SimpleImage.php');$image = new SimpleImage();$image->load('picture.jpg');$image->scale(50);$image->save('picture2.jpg');
?>

You can of course do more than one thing at once. The following example will create two new images with heights of 200 pixels and 500 pixels

<?phpinclude('SimpleImage.php');$image = new SimpleImage();$image->load('picture.jpg');$image->resizeToHeight(500);$image->save('picture2.jpg');$image->resizeToHeight(200);$image->save('picture3.jpg');
?>

The output function lets you output the image straight to the browser without having to save the file. Its useful for on the fly thumbnail generation

<?phpheader('Content-Type: image/jpeg');include('SimpleImage.php');$image = new SimpleImage();$image->load('picture.jpg');$image->resizeToWidth(150);$image->output();
?>

The following example will resize and save an image which has been uploaded via a form

<?phpif( isset($_POST['submit']) ) {include('SimpleImage.php');$image = new SimpleImage();$image->load($_FILES['uploaded_image']['tmp_name']);$image->resizeToWidth(150);$image->output();} else {?><form action="upload.php" method="post" enctype="multipart/form-data"><input type="file" name="uploaded_image" /><input type="submit" name="submit" value="Upload" /></form><?php}
?>

The code

<?php/*
* File: SimpleImage.php
* Author: Simon Jarvis
* Copyright: 2006 Simon Jarvis
* Date: 08/11/06
* Link: http://www.white-hat-web-design.co.uk/articles/php-image-resizing.php
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details:
* http://www.gnu.org/licenses/gpl.html
*
*/class SimpleImage {var $image;var $image_type;function load($filename) {$image_info = getimagesize($filename);$this->image_type = $image_info[2];if( $this->image_type == IMAGETYPE_JPEG ) {$this->image = imagecreatefromjpeg($filename);} elseif( $this->image_type == IMAGETYPE_GIF ) {$this->image = imagecreatefromgif($filename);} elseif( $this->image_type == IMAGETYPE_PNG ) {$this->image = imagecreatefrompng($filename);}}function save($filename, $image_type=IMAGETYPE_JPEG, $compression=75, $permissions=null) {if( $image_type == IMAGETYPE_JPEG ) {imagejpeg($this->image,$filename,$compression);} elseif( $image_type == IMAGETYPE_GIF ) {imagegif($this->image,$filename);} elseif( $image_type == IMAGETYPE_PNG ) {imagepng($this->image,$filename);}if( $permissions != null) {chmod($filename,$permissions);}}function output($image_type=IMAGETYPE_JPEG) {if( $image_type == IMAGETYPE_JPEG ) {imagejpeg($this->image);} elseif( $image_type == IMAGETYPE_GIF ) {imagegif($this->image);} elseif( $image_type == IMAGETYPE_PNG ) {imagepng($this->image);}}function getWidth() {return imagesx($this->image);}function getHeight() {return imagesy($this->image);}function resizeToHeight($height) {$ratio = $height / $this->getHeight();$width = $this->getWidth() * $ratio;$this->resize($width,$height);}function resizeToWidth($width) {$ratio = $width / $this->getWidth();$height = $this->getheight() * $ratio;$this->resize($width,$height);}function scale($scale) {$width = $this->getWidth() * $scale/100;$height = $this->getheight() * $scale/100;$this->resize($width,$height);}function resize($width,$height) {$new_image = imagecreatetruecolor($width, $height);imagecopyresampled($new_image, $this->image, 0, 0, 0, 0, $width, $height, $this->getWidth(), $this->getHeight());$this->image = $new_image;}      }
?>

Further Information

This script will eventually be developed further to include functions for easily sharpening, bluring, cropping, brightening and colouring images.

from:  http://www.white-hat-web-design.co.uk/blog/resizing-images-with-php/