I designed a box below the header to have an image…
The image has disappeared – but I cannot find how to edit it, delete it or anything of the like.
I have checked the page, I have checked the widget areas.
Can someone please help me find how to edit or delete this empty box on my front page?
The empty box you have is the slider. You can turn it off on Frontier Options -> Slider
If you want an image on that area you can use the Below Menu widget area. Add a Text Widget to it then add your code for the image.