First you must upload your pic's to a photo storage hosting site such as http://photobucket.com/
Once the pic's are at photobucket you can copy and paste the IMG code into your post a reply page. It will appear as a code. Click on the Preview button to see if it worked if you can see the pic then your all good an you can submit post.
If you'd like a better description check out the 7th post in this thread by jscott. He posted a great reply on posting pic's complete with visuals. [ Invalid URL Removed / p= t=89823 f=1 ]