It is very important to make sure, the images used in your domain are valid.
In the Dashboard, you will find an overview of how many broken images there are.
Broken in this sense means that they either have a header status code 4xx, 5xx or the status code is unknown. So in order to find the broken images, go to "Zoom" -> "Assets (Static Content)" -> "Images" -> "Status Codes" and then set the filter by clicking on the big status codes buttons "4xx (Client Error)", "5xx (Server Error)" and "Unknown".
Then you will get a list of all the broken images on your domain.