From the formatting help page:
Images can be added primarily by using the the editor toolbar button . This brings up a special interface that allows you to upload an image online (through the imgur hosting service) through us - even from your clipboard. Alternatively, it can be input similarly to adding a link:
- HTML
<img src="http://example.com/img.jpg">
- Markdown
![sample image](http://example.com/img.jpg)