Hello,
What are you trying to accomplish? Both are possible, and use different ways of attaching the image.
The first searches your existing media database for a coressponding image. The second imports a new image to your media database and attaches it to the product.
So, if your images are external and you have the URL, use that. If you already uploaded the images to your media gallery, use the names of these images to find and attach them.
