There is a problem with that though because when someone tries to save any of those images they will all be called 'image.jpg' because the script is called image.php. If you are using Apache you can be slightly clever by removing the file extension (and forcing it as PHP using the <File> directive in .htaccess) and putting the query string to make it look like a directory.
e.g. <img src="image/453.jpg">
Where you are actually calling image.php to find id number 453 in the database but as far as web browsers are concerned it's 453.jpg