Just use the image tag "
".
And in between the tags put the URL to the resource. I think for most people the challenge is how to get an image online. You need to upload it to a public node on the Internet that offers serving images over http.
Some people use free services like Photobucket. Some people already have space out there on the Internet, Like me, and we just upload the images there. You can upload using FTP if your space provider supports that protocol. Or some sites like Photobucket probably offer an HTTP based upload which is exposed through a webpage.
Regardless you need to establish space somewhere, upload the image to that space, and then reference the image at that space via an IMG tag here.
for example, I used an FTP client to upload my images to a web server located on the fmellish.shackspace.com domain. On that domain I uploaded my images to the folder aquarium/miages. So the following would be the address, (URL), to an example image.
http://fmellish.shackspace.com/aquarium/images/frogspawn_macro_high_speed_720.jpg
Now if I unclude that in an IMG tag, (this is a reefcentral convention, don't get hung up on the syntax, it's pseudo code that gets translated to HTML), I get the following
"[1MG]http://fmellish.shackspace.com/aquarium/images/frogspawn_macro_high_speed_720.jpg[/1MG]"
Now If I remove the quotes and use an "I" instead of a "1" it would actually work.
Cheers
Josh