Magento uses temporary images as placeholders until the permanent product images become available. A different placeholder can be uploaded for each role. The initial placeholder image is the Magento logo, which you can replace with the image of your choice.
Upload placeholder images
On the Admin sidebar, go to Stores > Settings > Configuration.
In the left panel, expand Catalog and choose Catalog underneath.
Expand the Product Image Placeholders section.
For each image role, click Choose File, find the image on your computer, and upload the file.
You can use the same image for all three roles, or you can upload a different placeholder image for each role.
When complete, click Save.
For information about image roles and recommended sizes, see Uploading Product Images.