Your picture must first be online somewhere, like photopoint.com, or your own homepage, or in some type of photo album, as shown here.
Then you have to cut and paste the exact location of the picture (the url)to the reply box, and use {img} before the url, and {/img} after the url. Except use [ and ] instead of { and }, and erase the "http://" (If I type out the exact code, it'll try to post the type as a picture, and won't work.) in case I thoroughly confused you, see: [url]www.ar15.com/forums/help/code.html[/url]
your photo code would look like this:
{img}wsphotofews.excite.com/016/wg/kE/wB/eL62890.jpg{/img}
Except of course use brackets instead of the { and }.
Hope this helps...