This is how it explains it in the FAQ section in the top left-hand corner of the page:
Can I post Images?
Images can indeed be shown in your posts. However, there is no facility at present for uploading images directly to this board. Therefore you must link to an image stored on a publicly accessible web server, e.g.
http://www.some-unknown-place.net/my-picture.gif. You cannot link to pictures stored on your own PC (unless it is a publicly accessible server) nor to images stored behind authentication mechanisms such as Hotmail or Yahoo mailboxes, password-protected sites, etc. To display the image use either the BBCode tag or appropriate HTML (if allowed).
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Basically, in one sentence do the following: [img]<Then type in the address of the source of the image>
* Note: The Img tag is the second last button of the above options when you make a reply (or you can simply type it in).
I hope I was of some assistance to you Jedda ;-)