I recevied an answer from support on this as below...
Thanks for writing. It is possible – you just need to have one image on each line of the CSV with the corresponding handle in that field. If, for example, a product has only one variant but three images, all the product data and the first image would be on line 1, while lines 2-3 would have the handle, the subsequent image URLs, and nothing else. Let me know if that works for you!