Uploading Shell or Anything Else.

So, my question is : How can I upload a file ( a shell of course ) on a web server, site, etc, if it doesn't have an upload function. I have to use the FTP server? For eg. : on imageshack or websites like that is simple, you have the upload button, but idk, here for example on null byte you don't have any buttons or functions for upload ( sorry for that eg, but I don't have any other sites in mind ). So.. ? I have to use FTP server right?