The featured image attachment id is stored in the post_meta table, with the meta_key
Use that post id to find the post meta with meta_key
The value of that is a serialized array with keys
sizes (an array), and
image_meta (an array).
file value is relative to the uploads directory for the WP install, and points to the original full-sized image. You should be able to calculate the URL from that.
sizes has an image size as its key (e.g., 'small', 'medium', 'large', 'thumbnail'), and the value is an array with keys
image_meta will contain things like exif/iptc image metadata from the image itself.