What web site? Presuming you're starting out with the free places, such as Angelfire, or Geocities (which serve ads to cover the costs and then some)... you'd either set up an FTP client (Angelfire), or login to the web site, and look for a link along the lines of "Upload File". Once done, you should be able to go to your website...such as
www.angelfire.com/aaltanahmad/image.jpg (Not exactly right, been a bit since I've used that...!) Their FAQs will also have extensive information on how to use FTP, and/or how to use the web site to upload files.