Can you use HTML without WYSIWYG off?
If yes, you can use "<img src='someimage.png'>"
Go to your profile, click edit and tick the box next to Use WYSIWYG editor where available.
Then you would post to a thread or start a new one normally.
When you are posting, click the picture icon and enter the url of the picture (Upload your picture to imgur.com) then click on done and click on the post button
srcattribute of the
imgtag. This is way simpler than it sounds:
<img src="http://puu.sh/9sTmu/1106d55291.png" />