You will have to post an image to an external image host (example: https://imgbb.com/) as you cannot post an image directly on Stockhouse.com
From imagebb, you must use the full image URL that is part of the HTML embed code.
Click on the link icon found on the text toolbar and then paste the URL there. There might be another window opening up asking you to enter description code. Please paste the URL on that box once again and it should work from there.