Several options to do this:
1. one-by-one per product (not recommended if you have a lot of products)
2. Settings > Files > upload images > copy URLs and add to each products (not recommended if you have a lot of products)
3. batch upload with the csv-files. If you have the URLs of your supplier, Shopify will download the images from your supplier website and add them to their server.
After the upload is done, export your product list to know where Shopify stored your images.
3. if you're dropshipping, do it via your app (e.g. Oberlo or whatever you're using) when importing.