You can import images at any time simply by using an import csv that has 4 columns
sku, base_image, small_image, thumbnail_image
Images can be pre-uploaded into the import folder if you have FTP access.
The import folder is where you put the web ready, pre-magento processed images that you name in the CSV when you want to bulk add images using the importer.
When you run an import, you specify which folder the images are in and therefore Magento goes to that folder to retrieve them and then pushes them out into cached image folders once it's processed them.
If all the images that were used in the 1.9.1 site were added using this method, then you can download the import folder contents and upload them to the 1.9.4 site import folder.
If you upload images in the admin, they get saved in the end up in the processed image folders /media/catalog/product/
The image names you see in the export are within those media/catalog/product/ folders. That means, to re-use them, you'd need to copy all those folders across from the 1.9.1 site to the 1.9.4 site.
If you go that route, you would download media/catalog/product/ and then upload the sub-folders of it, back up to media/catalog/product/ on the 1.9.4 site - make sense?
Magento indexes all images by taking the first and second character of the image name and creating folders named the same.
So, as an example, if the base_image for a SKU on your export is /w/m/wmbb1_1.jpg then you will need to copy
and so on.
In theory, if you copy all of media/catalog/product from 1.9.1 to 1.9.4 the product images will show up but copy the images before you run the import else it'll replace all the images with placeholders because they won't exist on import.
This is the Magento documentation on it for you - https://docs.magento.com/m1/ce/user_guide/store-operations/data-import-product-images.html
Hope that helps.