Yasmin here from Shopify Support!
A great way to do this is by using a CSV file. You can export your products, add image links in a spreadsheet, then import them with the new information. Here's our guide on using CSV files: https://help.shopify.com/manual/products/import-export
These are the steps you would go through for your specific case:
You can repeat these steps for each collection. Hope this helps!
By the way I am moving this topic to our general Shopify Discussion forums, I think it's a better fit there :)
So you're suggesting to go into your entire csv file and add 1 row for each product, of which we have thousands?
This is a terribly time consuming way to do things.
There has to be a better way.