Click little image-icon (a landscape) at the top of a blank text area;
Insert the URL of your picture in the URL field;
You can also upload a local image to the server, just press the button "Browse Server" next to the URL field, and you will have the option to upload an image, resize it if necessary and then insert it.
In order to complete the activity, press the "OK" button and then you should be able to see your picture inside the text editor;
When you are ready to publish your image, click on "SAVE".