First you need to upload them.
Hit Gallery (top left of this page), then go to upload
When you want to post them, simply highlight and copy the BB code under you pic, and paste it to the tread.
What I like to do is; open two tabs (windows) of the site. One tab will have the Gallery that has my pics, the other will have the tread I want to post them to.
Copy BB code from one to the other.
This is what your desk top will look like when viewing the pic you want to post. The BB Image Code is at the bottom.
Click this pic to see it bigger.