There are two ways of doing this.
When you hit "Post reply" or "New topic" there should be a little box below where you type your message that says, "Browse...". Click browse and find the picture you want to post.
Or you can upload your pictures to a hosting website like
www.photobucket.com or
www.imageshack.us If you upload there all you need to do is copy and paste the address and put [ i m g ] and [ / i m g ] around the address, minus the spaces. I like photobucket personally because they have links set up that you just copy and paste without having to type out the img code.
If you're still unsure, just send the picture to me and I'll post it for you.
Hope that helped.