The URL you cite is only possible if the server is set to provide a script of some sort (you seem to have discovered it's a PHP script already) when the directory http://www.site.com/images/ is requested, and the ?id=345435 part is passed as an argument to the script.

I don't think it's possible to enumerate the directory in this case, you need to know the ID of every image you want -- and the server may be set up so that the ID is different every time that image is to be requested. It's a sneaky way to attempt to prevent people from grabbing copyrighted pictures off a web site or linking to them from their own sites.