FOG uses an SQL database to keep track of clients as well as images and nodes. Thus it isn’t currently possible to ‘drag and drop’ images, sadly.
Now you want to use FOG as an ISO server, as in one that will just put ISO files in a directory available through HTTP? You don’t need FOG for that, hell, that’s way easier to do, just install a webserver and make a soft symbolic link to the directory with the ISO files in the root dir of the webserver.