Here is the very small code sample that will resize images as they are uploaded from a web form. Please read the comments in detail. They explain exactly what is going on, and where you should make changes for the destination directory and output sizes.
This script only supports the JPG file format. The compression algorithm for GIF images is protected by a restrictive licensing agreement that prevents the developers of PHP from including support for GIF images in the base platform.
Part I: The HTML Form
<form action="upload.php" method="post" enctype="multipart/form-data" >
<input type="file" name="uploadfile"/>
Part II: Getting at the file, resizing the image, and saving to the server. (upload.php)
// This is the temporary file created by PHP
$uploadedfile = $_FILES['uploadfile']['tmp_name'];
// Create an Image from it so we can do the resize
$src = imagecreatefromjpeg($uploadedfile);
// Capture the original size of the uploaded image
// For our purposes, I have resized the image to be
// 600 pixels wide, and maintain the original aspect
// ratio. This prevents the image from being "stretched"
// or "squashed". If you prefer some max width other than
// 600, simply change the $newwidth variable
// this line actually does the image resizing, copying from the original
// image into the $tmp image
// now write the resized image to disk. I have assumed that you want the
// resized, uploaded image file to reside in the ./images subdirectory.
$filename = "images/". $_FILES['uploadfile']['name'];
imagedestroy($tmp); // NOTE: PHP will clean up the temp file it created when the request
// has completed.