These screenshot guides explain it well:
http://forums.d2jsp.org/info.php?p=15 
You will need 15 posts to embed images, and it will not apply to posts made before you reached 15 posts, unless you edit them first. (You can only do so for 30 minutes)
You should also post this type of question in General Help, Site Suggestions is for changed to the site.
This post was edited by RaveTheGo on Nov 29 2012 10:18pm