Start a post, and add the image as you normally would.
Then switch to the source or html view and copy the code.
Then navigate to the widgets (it will notify you that you're not saving your changes, but we don't want to).
Drag a text widget to your sidebar if you have not done so already. Paste that code into the text widget and click save.
Here is a quick video http://www.youtube.com/watch?v=HCKLgt1sjH8&list=UUMmzF0F07MsauFO1k3ANgiA&index=2