If they are saved on your computer, just, while posting a message, click on the paper-clip icon. A box will pop up and you can browse your computer to find said pic for attachment.
If the picture is hosted on a website, just post the link.
every time there is a new post it brings it to the top of that forum page. So everytime you post in it or if anyone post in it we will all be able to see it.
When you go into a forum page you can see which thread has new post because till will be bolded compared to the others.