You need to have the picture published on the web - places like Photopoint or the like, or on your own website. Then you post a link to in in this format:
(img)www.photopoint.com/mypictureofawickedrifle.html(/img)
but replace the "(" and ")" with square brackets "[" and "]" like you would for changing the text.
The [red]Board Code[/red] explains all - click on those words next message you go to post.