First you should realise, you need at least 10 reputation points to be able to use images in your post. You can check the accompanying reputation page, which also explains how to use images in your post at the bottom.
From that page:
How do I add images and links to my posts?
We use Markdown to format posts, so there are a variety of ways of adding links:
- bare urls
<a href="http://example.com">example link</a>
- the editor toolbar button insert link toolbar button
Images can be added similarly:
- the editor toolbar button insert image toolbar button
Just to give you some graphical aid, which I always love: