it's super easy:

- 1 -

Find the URL of the picture. The URL is what you would see in the address bar of your browser or when you right click a picture and choose something like "copy link address". (see image below)

- 2 -

Paste the URL into the text area where you write the post.

That's it, your done. You can use the [Preview Post] button to test out if all worked out fine.

Here's an image that shows the URL bar of a browser: