There are three ways you could do this:

1. Upload all into the Media Library

The most tiresome way, IMHO, is to upload all the images you got via the media library.

2. Use a plugin

You can use plugins like Add From Server to import your images easily to the Media Library

3. WP-CLI

Easiest way once you got WP-CLI installed. Run wp media import to import all images easily.
Quoting an example:

# Import all jpgs in the current user's "Pictures" directory, not attached to any post
wp media import ~/Pictures/**\/*.jpg