Shopify has a possibility to update fields using a CSV file. This is useful if you want to update a lot of images.
Shopify will download these images from a remote server if necessary, so you can provide URLs of your choice.
You only need to provide the values you want to update, along with some core values.
These are the ones I found necessary to include for the Variant Image update:
(The Variant Inventory Qty is probably not necessary – I wanted to test adjusting the inventory quantities, too).
Here are the fields for copy & pasting for your convenience:
|Variant Inventory Qty
First you export the product, you get rid of all unnecessary fields.
You edit the Variant Image field (note this is NOT the Image field).
I suggest to use LibreOffice Calc for this, and to name your files with <sku>.jpg, and upload them to a directory on your webserver.
This way you can quickly add the images using a LibreOffice Calc formula
then you drag the formula down, so that it will update to the cell rows.