If I face such scenario, I would navigate to "Administration > Set-up and management > Personalization > Elements > HTML" tab and check whether "**Enable HTML descriptions and properties**" option is enabled.
Please be informed that you could add the image as a link and try assigning the Property Group to the view again.
At the moment, adding customizable iFrame to Data Catalog is unavailable and will be included in future updates.
If you require further assistance and if you are a valid support user, you could create a support case on Denodo Support Site so that our team will assist you.
Hope this helps!