On the drop down menu and click on the one that says "grab the HTML/BBCode" and chose the BBCode.
Select the size of the picture you want to post in the drop down menu directly above where you chose the BBCode option.
Copy the info that appears in the code box then paste it into the body of your message. It will look like this;
Once you have pasted the code, highlighted in blue, into the message body you will get this;