Same as the other place Steven (same forum software)
Either use the "Additional Options" button when posting to attach a small jpeg picture, or link to an externally hosted picture (like photobucket for example) using the "Insert Image" icon above (the one that looks like a picture frame.
When linking to external pictures, limit them to 600 wide as they fit the page better:
{img width=600}paste url here{/img} but use square brackets