To add a photograph to a customer's file you can go to  Customers -> Customers and press  to access the cuomer's file.

Once inside the customer's file, click on the picture in the left upper corner (marked with a red box on the following screenshot).

Then you'll see a new window, where you'll be able to take a picture of the customer right there with your webcam, or upload a picture from your computer if you already have one.