Making a reservation for a product image that is coming soon is a great idea if you haven’t finished the major one yet. The replaced image is called as Placeholder. This means store owners will use a temporary image on the product list page until the offical product image is ready to appear.
By the default, the uploaded placeholder image is often the Magento logo, but you are also authorized to apply other images as your need. The placeholder image is displayed on the catalog page as the following:
So what is the fast way to upload the placeholder image via Magento Admin?
Stores > Settings > Configuration.
Catalog, choose Catalog.
Product Image Placeholderssection.
Choose Fileto upload the placeholder image. You can use the same or the different images for each role.
Save to complete.
Reference: Magento 2 user guide
It comes to the end of tutorial: How to upload Placeholders in Magento 2.