Directly Upload and Assign Images

When editing a product multiple images can be uploaded simultaneously.

Assign Via Product Image Manager

Another method of of assigning images is by initially uploading them to the product image manager. Then later on when you are editing a product you can select the images and assign the images you uploaded earlier via the popup gallery box.

Assign Product Images Via Bulk Actions

The third method allows you to assign product images without having to go into a product each time as you can remain in the product manager page and use the bulk actions interface.