See Greg's reply to my question on this from Oct 17, 2000. I will now paste in Greg's instructions....
You should be able to point to a .jpg just as you would any other file. A URL doesn't imply an HTML page - only a complete path to a file name in several parts:
1) The protocol http: is hypertext transport protocol but there are other valid protocols that use URLs, like HTTPs and FTP.
2) The server name (including the domain where the server lives: photos is the server name and yahoo.com is the domain in this example: //photos.yahoo.com
3) The path to the file (an HTML file, a CGI Script, a JPEG image, etc.) /mydir/myphoto.jpg