Could it be that your image is hosted locally and when you try to reference it on the web it just doesn't load? I imagine in if you look in inspector you will get a failure to load. I think you could solve this by hosting the image somewhere online and targeting it that way.
It seems to me that this is a limitation of the Single Integration API. If you use the Single configurator function from the dev-hub page and select a sheet, it shows you exactly what the API will generate. I just tried this with a new app for which I created a page title and the title was not displayed. So, if you wanted to display the page title, you would have to include this outside the iframe using standard HTML.