Only allowin images to be uploaded

I'm a little stuck on how to make it to where only images can be uploaded and not all files. Thanks in advance!

<?php

// Your file name you are uploading
$file_name = $HTTP_POST_FILES['ufile']['name'];

// random 4 digit to add to our file name
// some people use date and time in stead of random digit
$random_digit=rand(0000,9999);

//combine random digit to you file name to create new file name
//use dot (.) to combile these two variables

$new_file_name=$random_digit.$file_name;


//set where you want to store files
//in this example we keep file in folder upload
//$new_file_name = new upload file name
//for example upload file name cartoon.gif . $path will be upload/cartoon.gif
$path= "upload/".$new_file_name;
if($ufile !=none)
{
if(copy($HTTP_POST_FILES['ufile']['tmp_name'], $path))
{




echo "Successful<BR/>";

//$new_file_name = new file name
//$HTTP_POST_FILES['ufile']['size'] = file size
//$HTTP_POST_FILES['ufile']['type'] = type of file
echo "File Name :".$new_file_name."<BR/>";
echo "File Size :".$HTTP_POST_FILES['ufile']['size']."<BR/>";
echo "File Type :".$HTTP_POST_FILES['ufile']['type']."<BR/>";
}
else
{
echo "Error";
}
}
?>