In order to bulk import images you need to follow those specific steps : 

1) Be in debug mode (go to settings--> at the bottom you select "Activate the developer mode". 

2) Go to the "Document" App and upload your images

3) Export the attachment_id/local_url and the name

4) Go to "Attachments"

5) Put all those images (via an import or manually) to "Is public document"

6) Go to "Products" and export their External ID 

7) Add your domain name in front of the Url you exported to make the right link to the image

8) Match Products and images in an excel

9) Import them 
Video : https://watch.screencastify.com/v/Fxe80jfk6dBWqI8aa51U

To add extra image

1) Be in debug mode

2) Go to "Models" and select "Product Image"

3) Create a menu for this model

4) Go to this model

5) Create a new file with url's of extra images and product you want to link it to

6) Import by selecting the "Allow matching with Subfields" 



Video : https://watch.screencastify.com/v/cKOGvDyprnsb7dJug2Xj


It seems that the upload limit is around 60 MB per import. In order to reduce your image size without losing the quality, you can use websites such as https://tinyjpg.com/

UPDATE : Follow this video : https://drive.google.com/file/d/1PrWetqwEWHlxMf--rHBW-g3tMKLij9EU/view