In Magento 2 format of CSV file has been changed. The main difference is:
Magento 1 field values can allocate multiple cells in a column.
Magento 2 field values can allocate only one cell. Delimiter is used for separation.
As for images, you must use:
base_image - for base image
small_image - for small one
thumbnail_image - for thumbnail
additional_images - for all other
images in format(comma-separated):
To understand format better export some products and take a look into received CSV file.
Answer to comment.
No, it's not a bug.
Every column represents some product EAV attribute.
media_gallery contains all assigned images, other attributes just assign role(base, small or thumbnail) to image.