In your CSV file, the header for your extra images must be titled : media_gallery
then is should contain the following structure for additional images:
sku_01.jpg::sku_label;/sku_02.jpg::sku_label;/sku_03.jpg::sku_label;
(note 1: in this example the separator is ; so make sure it's the case in your magmi settings)
(note 2: image, small_image, thumbnal, image_label, thumbnail_label must also be in the CSV. In other words, you must not include the first image inside media_gallery, but only the additional ones.
More info from the magmi wiki: http://sourceforge.net/apps/mediawiki/magmi/index.php?title=Image_attributes_processor#media_gallery_.28extra_images.29_column_syntax*
I hope this helps.
edit - your images must be located in media/import
Are your images taken from a remote url ? Or are they on your magento server ?
The image processor is required if you wish to import images against your products. This plugin has several advanced features that are explained below.
This document refers to Image Attributes Processor Version 1.0.25
Base Images
The three base images can be imported by using the attribute names image, small_image and thumbnail
sku,image,small_image,thumbnail
00001,/00001.jpg,/00001small.jpg,/00001thumb.jpg
00002,/image1.jpg,/image2.jpg,/image3.jpg
The images will be imported from the folder defined in the "Image search path" setting shown in the configuration screen (entered after you enable the plugin)
Image search path
The "Image search path" field specifies the folder that your original images are stored in. The files are copied from here to the media/catalog/product image folders during the import process. This search path can be relative or absolute.
if you enter a path that does not include a leading forward slash, the value will be prefixed with the "Filesystem Path to magento directory" setting shown at the very top of the Magmi configuration screen (next to the database settings)
This normally contains the value "../.."
If you look at the two screenshots shown above, this will give a combined search path of ../../media/import
This search path is relative to your Magmi home folder (and not the Magento home folder). The image names included in your CSV file should be prefixed with a forward slash. They will directly appended to the end of the search path derived above
Absolute server path
Alternatively you can include a full server path for the images. To do this, include a leading forward slash when you enter the "Image search path" in the configuration screen.
/var/www/media/import
When you include a leading forward slash, the "File system path" is not appended and the raw value you enter will be used instead.
Note : The path does not need to be part of your Magento directory structure. For example, the path could be /tmp – or any other location on your server hard drive.
More Column Headers
sku
description
image
attribute_set
weight
media_gallery
type
tax_class_id
qty
store
price
is_in_stock
name
thumbnail
visibility
att_eby_title
small_image
Best Answer
The issue here was I was renaming the images with
{item.name}-{item.sku}.jpg, so both images were getting the same name and being overwritten. I just removed the image renaming from the magmi config and it worked.