It needs to be hosted on a website. If it is already on a webpage (eg airliners.net) you can use the url from that otherwise you need web space.
Copy the full url, including the http or ftp into your message, eg
http://www.csharpprogrammer.com/files/improved4.jpg
then highlight the url and click on the
Img button above and BYU, you get
If you quote this message you will see the img tags around the url
on the whole http works fast than ftp.