You have to upload your pictures onto the web.

A good place to do this is somewhere like...

http://www.photobucket.com

It's free and they explain it pretty well for you there...

They will then give you a link to direct link your photo...

Once you have that you come here and either click the image of th mountains at the top right of the posting window when you are making a message and paste or type the link in there or...

you can manually do it by doing [img]insert image link here[./image]
(without the . in front of the / on the close bracket.