Magento 2 Image import functionality is supported by Out of Box Magento. Its default Feature of Magento, you can import a list of product images via CSV from admin panel.
If you want to only import images for a product, You need to create a CSV file and add columns for different images.
List of the possible column for import images.
sku => Product Unique SKU
base_image => pic1.jpg (Product main image in product page)
base_image_label => pic1.jpg (Product main image label)
small_image => pic1.jpg (Product small image in cart page)
base_image_label => pic1.jpg (Product small image label)
thumbnail_image => pic1.jpg (Product thumbnail image in listing page)
thumbnail_image_label => pic1.jpg (Product thumbnail image label)
swatch_image => pic1.jpg (Product swatch image in listing/detail page)
swatch_image_label => pic1.jpg (Product swatch image label)
additional_images => pic1.jpg,pic2.jpg,pic3.jpg (Product additional image for product page gallery thumbnail)
additional_images_label => pic1.jpg,pic2.jpg,pic3.jpg (Product additional image label by comma separated)
Default Magento takes the path for images from pub/media/import folder so before import images, You must check your all the images are available in import folder.
For Add Custom Path to import an image:
If you want to add your custom path for insert images, You need to add your path inside the Images File Directory text field value in System -> Import
Product Import Settings.
If all of your images under the root var/import folder, you need to set above field value as var/import and your images will be taken from var/import directory.