there are two ways to do that, one is open an ftp account, but unless it is neccesarry i would't use it, because you have a username and password which can be directrly attacked.
Another way is using Uploads via HTTP. For such a purpose there are many solutions you can find in the phpbuilder Code-Library or at zend ... eg: http://www.phpbuilder.com/snippet/download.php?type=snippet&id=549
have fun.
ali