Hello!

There is this one external contact from a federated domain. I can chat with him and see his online-status.
BUT:

His photo is being shown only when I use Lync2011MAC but is NOT shown when using Lync2013basicWin!!!

Is there any logical explanantion why this happen on my Lync2013 setup?

Did you test that with other Lync client?

Did this issue happen to other users?

Firstly, you need to check where the picture of federated user from. If it is stored in AD DS, then you wont see the picture. If the federate use a publicly accessible URL, try to browser the picture URL on the computer where Lync 2013 basic is on to check you can access the picture successfully.

I tested it with diffferent PCs but yes, it was always the same Client Lync2013Basic.

I have to assume that it is a photo coming from an URL. but how do I know for sure? I dont see any possibility to get to know this from my side.

There is no difference between clients reg. availiability of a photo-url as both computers sit on the same network. There is nothing only the mac could reach but the PCs could not..

To check the source of federated users picture, you need to check with the user.

You should check this first, then we will know whether this issue is related with Lync 2013 basic or your Mac client.