When you found the pic you wanted to post, go to Post Reply. At the bottom, you will see a Browse button in the Attached File section. Click on that button to upload your picture.
Once it has been uploaded, you need to find the URL of the picture you just uploaded. What I like to do is submit my posting first, and then edit my posting. When you are in the edit screen, you will see that there is a link to your file in the Attach File section. Click on it to find out the URL of your picture.
Once you got the URL, put that in code....like this [img ] [/img ] , but with the URL of your picture in between the two bracket. I think that would do the trick.
I hope this help, because I am not that good at explaining stuffs.