Yappari shows pictures assigned to the contacts but that YOU assigned. Pictures from Skype, or GTalk or any other IM integrated in the N900 won't be shown.

A simple way to see the real contact pictures is turning off the IM Availability (signing off) and then to go to your address book. If you assign a picture there then Yappari will show that.

Anyway that's just temporary until the Whatsapp profile pictures is implemented (but if a contact hasn't configured a picture in Whatsapp Yappari will show the contact address book pic).