If you want to setup a basic product image for a released item then you can do that from the fact box on the released item. It’s worth noting that the images are stored based on how the document handling is setup. So you might want to consider this to manage storage size/location.
It’s also worth noting while the image is stored in document handling the link to the released product is through a table called EcoResProductImage.
Here is a quick overview.
D365 July 2017 Update 8 (7.0.4565.16211)