i have made one entry but now i see 2 adressbooks , which one ist the correct one ?
You can use the one you like. You can see the URLs of both ones, so if you have questions about those address books, please contact your CardDAV server vendor (Synology). I’d use the second one because it contains the user name instead of an UID.
what must i do to sync also the picture of the contact ?
Nothing, contact photos are synchronized automatically.