When you access a stockpile's 3D view, your browser gets all the necessary data from the server and renders it on your computer. At that moment, your browser takes a screenshot of this stockpile and saves it for the report.
Stockpiles that have not been viewed in 3D do not have a saved image yet. If a stockpile's image is missing, all that needs to be done is view the stockpile in 3D and create the report again.
We understand this is a tedious step and we are working to eliminate. However it is good practice to inspect all stockpiles in 3D to verify floors were estimated appropriately.