When working on templates for clients, I'm typically inserting temporary image content to get through training. 

In this case, I'm using the default Squarespace "Default Image" to show the client admin: "you need an image here". 

Simply, on mouse hover, the user would see an image is needed when they see the image I've provided.

Another class could be selected to normally display the helper image, or a logo could be added to replace the default image JUST IN CASE the site is launched with this "hack" in place.