How to insert image path in database to display product image in wp-admin?

While this is theoretically possible it is going to be very unpleasant to get right.

The “image” in WordPress view would be attachment post with considerable amount of metadata, recording information about physical file for image itself, image sizes, EXIF info from image, and so on.

You should really be using API for task like this, I would start with looking at media_handle_sideload().

PS wp_insert_post() for creating normal posts would be practical as well.