There is no default way to import bulk pictures of products. May be you need to upload picture manually or can create a plugin that import data to Picture table and map it with product Id.
I think you already know how PorductId map with PictureId.
You can use Bulk Import Plugin with following features:
1) Bulk Upload of Product, Categories, Manufacturers, Product Attributes. 2) Bulk Upload of pictures in Product. 3) Bulk Import of new categories, manufacturers, product attributes in product. Once new categories, manufacturers and product attributes are imported the products can be imported in bulk mapping with new categories and manufacturers etc. 4)User can upload 10,000 records in 120 minutes.