The COTROnline Text Editor already provides an insert image function, so why use a shortcode here? 

Shortcode attributes will allow you control over the layout on the page, turning images into links, and setting margins. You can also use this function to insert images from elsewhere on the web without downloading them. *But make sure to give proper attribution!

The above image is linked from the accordion page using the image link address as opposed to inserting the image directly into the page. 

In your text editor, it looks like this: