First step is to find a web site to host the image.

I use [imgur](http://imgur.com/) but there are lots of other sites.

Once the image is hosted

**How do I add images to my posts?**:

 - HTML `<img src="http://example.com/img.jpg">`
 
 - Markdown `![sample image](http://example.com/img.jpg)`

 - the editor toolbar button insert image toolbar button

Taken directly from the [faq](https://askubuntu.com/privileges/new-user)