It should posted like this with the extension at the end otherwise It won't show up directly:
[img]https://i.imgur.com/JwsVC1k.png[/img]
Additionally, It's impossible to post images unless you are a junior member (+30 Activity) and for videos, It's impossible to post them directly, there has been some occasions where theymos (admin of the forum) posted videos embedded but he used HTML which is disabled in normal users case.