Ok first off you need to have an image online. I suggest creating a photobucket account, see: http://photobucket.com here you can then manage and upload pictures to be available on the internet.

Once you have done this create a new post, on the "format message" section you will see a Click on this to insert 2 image tags "[img][/img]" then type the full address (url) of your your picture inbetween the square brackets. For example

[img]http://www.british-bodybuilding.co.uk/images/Zackc_small.jpg[/img]

would post this picture: