We agree that adding photos is important as it adds life to your profile. However, some people take their own time to display photos on social platforms.
Once they are confident that adding photos will get them more connect, they go ahead & do that on their own. Most of the members do upload their photos during the registration itself.
On our part, we keep notifying these members about the benefits of adding photos to prompt them to add it sooner. In the meantime, you can always prefer to connect profiles with photos.