Each product has a main image that you add when creating/importing the product. In case you've skipped the step, you can add or reset the image the following way:
1. Navigate to Catalog on the left sidebar.
2. Using catalog search, find the product you'd like to add an image to.
Tip: In the Image filter select No, and click Search. Now that you filtered products without pictures, go ahead and add them!
3. When the needed product is found, click the Image icon.
4. You'll be redirected to the product information page. Scroll down to the Images and click the Upload Image. Note that the image size should be 1024 X 1024 PNG.
5. That's it, make sure to scroll down and Save Product.