WhatsApp and many other messaging apps, doesn't provide the contact icon when there are unread messages from several contacts or groups. in that case they are only providing a generic app icon.
- If you have 2 unread messages from contact A - WhatsApp will provide the contact icon of contact A.
- Then, if you'll get additional unread messages from contact B - WhatsApp will provide a generic WhatsApp icon.
- Then, if you'll mark as read the 2 messages from contact A - WhatsApp will provide now the contact icon of contact B.
This is how those apps creates the notifications - NiLS cannot access the individual app contact icons - it fully depends of the icons that provided with the notification itself.
As a reference you can see the original notification on Android notifications bar and see that it doesn't have those contact icons either.
Unfortunately it is not possible to solve it on NiLS's side. I hope that with the new Android Nougat individual messages capabilities - messaging apps will start to provide contact icons for each notification.
If you have any ideas or thoughts about it, please contact firstname.lastname@example.org or post comment here.