The Extra product images are handled through media_gallery column.
Since several images can be set in this column,a specific syntax has been set to handle labels.
- The Extended syntax with :: for setting labels ONLY APPLIES TO media_gallery column
- TO SET BASE IMAGE LABELS USE BASE IMAGE LABEL RELATED COLUMNS.
For more details, please have a look at below links:
http://wiki.magmi.org/index.php?title=Image_attributes_processor#Example_.28with_no_labels.29:
http://magento.stackexchange.com/questions/13319/magmi-import-extra-images-how
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