In order to post a picture on the forum you need to have it hosted online on a web server. There are several free ones out there, these are a few of the better ones.
Hunt101 Photobucket UpitAll
All of them are free and for the most part you register an account just like you do to join a forum. Most will also give you the necessary url and encoding to post your pictures in your post.
Once you have uploaded your pictures and copied the url for the picture you are ready to post it in your thread.
There are three ways to do this.
1) If your photo hosting site gives you a link to paste that has the proper tags you can just paste it in to your post. It should look something like this:
[/img] but without the spaces.
2) If you have the url for the picture you can manually add the necessary tags yourself. simply type [img] before the url of the picture and [/img] after it. It should end up looking like this:
[/img] Again make sure you do not have spaces between the tags and the url or in the url itself.
3) If you have the url you can click on the little image that looks like mountains and a sun, above the window where you type in your post or reply. This will open a pop-up that asks for the url of your picture. Paste in the url for your image and click ''OK'.
With any of these three methods your picture should show up in your post as an image.