Alright, If you want to post a pic from your computer....use a site like ImageShack® - Hosting
or Image hosting, free photo sharing & video sharing at Photobucket
to UPLOAD the pic. And from there, your have to right click the pic, and go "Copy Image Location", and then go to the advanced reply section, look around at the top toolbar, find the "Insert Image" button, and then paste the link in there. An easier way would be to insert the link between the tag [*IMG][*/IMG](Without the *).
And if you just want to post a pic from the net, the same thing except you don't have to upload it. Hope I helped.