Attach image from folder

Hi,
I’m working on an classifieds site and we have a large database of images with buildings. When a user is listing an ad the website needs to be able to attach our images to that post accordingly to some custom fields values. What I was trying was to insert in the database a new row for the image attachment with the new post as parent.
But I didn’t find the correct way to link the image path to the new inserted field in wp_posts.
So my question is if I need to add a new attachment in the database (from the mysql point of view ) how would I go by defining the image path?
If anybody has a different way of going I am as receptive as possible to new ideas on how to do this.
Thank you a lot.
Jnhghy