Here follows the php code to process post data.

The code contains image extension and size  verification as well as uploading and resizing image.

// This variable is used as a flag. The value is initialized with 0 (meaning no error found) 
 and it will be changed to 1 if an errro occures. If the error occures the file will not be uploaded.
 $errors=0;
 // checks if the form has been submitted
 if(isset($_POST['Submit']))
 {
 //reads the name of the file the user submitted for uploading
 	$image=$_FILES['image']['name'];
 	// if it is not empty
 	if ($image) 
 	{
 		// get the original name of the file from the clients machine
 		$filename = stripslashes($_FILES['image']['name']);
 		
 		// get the extension of the file in a lower case format
 	 	$extension = getExtension($filename);
 		$extension = strtolower($extension);
 		// if it is not a known extension, we will suppose it is an error, print an error message 
 		and will not upload the file, otherwise we continue
 		if (($extension != "jpg")  && ($extension != "jpeg") && ($extension != "png"))	
 		{
 			echo '<h1>Unknown extension!</h1>';
 			$errors=1;
 		}
 		else
 		{
 			// get the size of the image in bytes
 			// $_FILES[\'image\'][\'tmp_name\'] is the temporary filename of the file in which 
			//the uploaded file was stored on the server
 			$size=getimagesize($_FILES['image']['tmp_name']);
 			$sizekb=filesize($_FILES['image']['tmp_name']);

 			//compare the size with the maxim size we defined and print error if bigger
 			if ($sizekb > MAX_SIZE*1024)
 			{
 				echo '<h1>You have exceeded the size limit!</h1>';
 				$errors=1;
 			}

  			//we will give an unique name, for example the time in unix time format
 			$image_name=time().'.'.$extension;
 			//the new name will be containing the full path where will be stored (images folder)
 		 	$newname="images/".$image_name;
 			$copied = copy($_FILES['image']['tmp_name'], $newname);
 			//we verify if the image has been uploaded, and print error instead
 			if (!$copied) 
 			{
 				echo '<h1>Copy unsuccessfull!</h1>';
 				$errors=1;
 			}
 			else
 			{
 				// the new thumbnail image will be placed in images/thumbs/ folder
 				$thumb_name='images/thumbs/thumb_'.$image_name;
 				// call the function that will create the thumbnail. The function will get as parameters 
 				the image name, the thumbnail name and the width and height desired for the thumbnail
 				$thumb=make_thumb($newname,$thumb_name,WIDTH,HEIGHT);
 			}}	}}

  //If no errors registred, print the success message and show the thumbnail image created
 if(isset($_POST['Submit']) && !$errors) 
 {
 	echo "<h1>Thumbnail created Successfully!</h1>";
 	echo '<img src="'.$thumb_name.'">';
 }

  ?>