You can't at the moment because you're still a newbie.
You can either buy a copper membership[1] or wait until you become a Junior member and then:
1. Upload the picture to a hosting provider like imgur.
2. Post the link inside of an image tag:
Make sure the URL ends with the image extension, otherwise it won't show.
And to make it clickable, just put the above inside of an URL tag:
[url=URL_HERE][img]imag_url.jpg[/img][/url]
[1]
https://bitcointalk.org/index.php?action=credit;promote