Importing images with multiple rows will work only for updating products, which means if you are creating new product multiple rows will be ignored. but if its an update you can add many images. But make sure multiple rows should be replica of original row with image name changed, if you just use sku in multiple rows there is chances to reset other information. So the option is copy first row and change image_filename in column.
For the category images import, I have created a issue for category image import. It will fix soon and you will get in next version.