You can't simple because you are still a "Newbie". You need to be at least Jr. Member to be able to post images.
See that the image is shown in the quote above because I'm already a Sr. Member.
Disabled images get transformed into links automatically. When the poster becomes a Jr member, the images that he tried to post previously will automatically be re-enabled (though there might be a few minutes of delay).
https://bitcointalk.org/index.php?topic=1172918.0