@Unifato brilliant idea! thanks!
We had the same problem recently and we decided to ease our future work by creating a small script that can speed up the process. What this script does it basically: Uses your exported xml file to find all images, use placehold.it to create the same sizes images and then put it in your wordpress uploads directory, then replacing all the links. That way when a customer imports the file all the images will be placehold.it safe ones.
You can find the script in Github: https://github.com/denchev/Wordpress-to-placehold.it
Please fill free to contribute if you find it useful.Kind regards
It is a great sharing. Thanks a lot