How can I assign a photo to a contact?
In Threema, you can't define the profile picture that will be associated with you on other people’s contact lists. Rather, you can define the profile pictures of your contacts yourself.
If contact sync is enabled, Threema uses the profile pictures of your device's address book. If a contact in Threema isn't linked to your device's address book, you can set the profile picture directly in Threema.