Lync 2013 Profile picture not visible for customers outside my network

We have installed Lync 2013 and everything works fine.
We are using a program to implement the profile pictures in Active Directory.

Now eveybody inside our organisation have a profile picture in Lync and everybody can see the other his profile picture.
The customers we have (outside our organisation) can not see the profile pictures.

Is it possible that they can see the pictures? and how and where must i do this?

Bart